chore: add testing section and format contributing file (#1574)

* Add testing section to CONTRIBUTING.md

---------

Co-authored-by: Waldemar Quevedo <wally@nats.io>

CONTRIBUTING.md

@@ -1,6 +1,8 @@
 # Contributing
 
-Thanks for your interest in contributing! This document contains `nats-io/nats.go` specific contributing details. If you are a first-time contributor, please refer to the general [NATS Contributor Guide](https://nats.io/contributing/) to get a comprehensive overview of contributing to the NATS project.
+Thanks for your interest in contributing! This document contains `nats-io/nats.go` specific contributing details. If you
+are a first-time contributor, please refer to the general [NATS Contributor Guide](https://nats.io/contributing/) to get
+a comprehensive overview of contributing to the NATS project.
 
 ## Getting started
 
@@ -10,36 +12,69 @@ There are three general ways you can contribute to this repo:
 - Reporting a bug or regression
 - Contributing changes to the source code
 
-For the first two, refer to the [GitHub Issues](https://github.com/nats-io/nats.go/issues/new/choose) which guides you through the available options along with the needed information to collect.
+For the first two, refer to the [GitHub Issues](https://github.com/nats-io/nats.go/issues/new/choose) which guides you
+through the available options along with the needed information to collect.
 
 ## Contributing changes
 
-_Prior to opening a pull request, it is recommended to open an issue first to ensure the maintainers can review intended changes. Exceptions to this rule include fixing non-functional source such as code comments, documentation or other supporting files._
+_Prior to opening a pull request, it is recommended to open an issue first to ensure the maintainers can review intended
+changes. Exceptions to this rule include fixing non-functional source such as code comments, documentation or other
+supporting files._
 
 Proposing source code changes is done through GitHub's standard pull request workflow.
 
-If your branch is a work-in-progress then please start by creating your pull requests as draft, by clicking the down-arrow next to the `Create pull request` button and instead selecting `Create draft pull request`.
+If your branch is a work-in-progress then please start by creating your pull requests as draft, by clicking the
+down-arrow next to the `Create pull request` button and instead selecting `Create draft pull request`.
 
-This will defer the automatic process of requesting a review from the NATS team and significantly reduces noise until you are ready. Once you are happy, you can click the `Ready for review` button.
+This will defer the automatic process of requesting a review from the NATS team and significantly reduces noise until
+you are ready. Once you are happy, you can click the `Ready for review` button.
 
 ### Guidelines
 
 A good pull request includes:
 
-- A high-level description of the changes, including links to any issues that are related by adding comments like `Resolves #NNN` to your description. See [Linking a Pull Request to an Issue](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue) for more information.
-- An up-to-date parent commit. Please make sure you are pulling in the latest `main` branch and rebasing your work on top of it, i.e. `git rebase main`.
-- Unit tests where appropriate. Bug fixes will benefit from the addition of regression tests. New features will not be accepted without suitable test coverage!
-- No more commits than necessary. Sometimes having multiple commits is useful for telling a story or isolating changes from one another, but please squash down any unnecessary commits that may just be for clean-up, comments or small changes.
-- No additional external dependencies that aren't absolutely essential. Please do everything you can to avoid pulling in additional libraries/dependencies into `go.mod` as we will be very critical of these.
+- A high-level description of the changes, including links to any issues that are related by adding comments
+  like `Resolves #NNN` to your description.
+  See [Linking a Pull Request to an Issue](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
+  for more information.
+- An up-to-date parent commit. Please make sure you are pulling in the latest `main` branch and rebasing your work on
+  top of it, i.e. `git rebase main`.
+- Unit tests where appropriate. Bug fixes will benefit from the addition of regression tests. New features will not be
+  accepted without suitable test coverage!
+- No more commits than necessary. Sometimes having multiple commits is useful for telling a story or isolating changes
+  from one another, but please squash down any unnecessary commits that may just be for clean-up, comments or small
+  changes.
+- No additional external dependencies that aren't absolutely essential. Please do everything you can to avoid pulling in
+  additional libraries/dependencies into `go.mod` as we will be very critical of these.
 
 ### Sign-off
 
-In order to accept a contribution, you will first need to certify that the contribution is your original work and that you license the work to the project under the [Apache-2.0 license](https://github.com/nats-io/nats.go/blob/main/LICENSE).
+In order to accept a contribution, you will first need to certify that the contribution is your original work and that
+you license the work to the project under
+the [Apache-2.0 license](https://github.com/nats-io/nats.go/blob/main/LICENSE).
 
-This is done by using `Signed-off-by` statements, which should appear in **both** your commit messages and your PR description. Please note that we can only accept sign-offs under a legal name. Nicknames and aliases are not permitted.
+This is done by using `Signed-off-by` statements, which should appear in **both** your commit messages and your PR
+description. Please note that we can only accept sign-offs under a legal name. Nicknames and aliases are not permitted.
 
 To perform a sign-off with `git`, use `git commit -s` (or `--signoff`).
 
 ## Get help
 
-If you have questions about the contribution process, please start a [GitHub discussion](https://github.com/nats-io/nats.go/discussions), join the [NATS Slack](https://slack.nats.io/), or send your question to the [NATS Google Group](https://groups.google.com/forum/#!forum/natsio).
+If you have questions about the contribution process, please start
+a [GitHub discussion](https://github.com/nats-io/nats.go/discussions), join the [NATS Slack](https://slack.nats.io/), or
+send your question to the [NATS Google Group](https://groups.google.com/forum/#!forum/natsio).
+
+## Testing
+
+You should use `go_test.mod` to manage your testing dependencies. Please use the following command to update your
+dependencies and avoid changing the main `go.mod` in a PR:
+
+```shell
+go mod tidy -modfile=go_test.mod
+```
+
+To the tests you can pass `-modfile=go_test.mod` flag to `go test` or instead you can also set `GOFLAGS="-modfile=go_test.mod"` as an environment variable:
+
+```shell
+go test ./... -modfile=go_test.mod
+```