Move all tests to a separate module to reduce dependencies

[Yiling-J]

Fixes #71

This PR moves all tests to a new module called tests, reducing dependencies in the main module. Additionally, it includes the following updates:

• Creates a Go workspace.
• Updates the Makefile test command to cd into the tests folder first.
• Updates the CI lint step to check both the txdb and tests modules. This step is a little tricky, but fortunately, golangci-lint-action provides an example.
• Updates the CI test step to use ./tests as working-directory.

To verify:

• I confirmed that the lint CI works by manually introducing errors in db.go and tests/db_test.go.
• I confirmed running go test in tests module will use the db.go file in parent directory by manually introducing error in db.go:

go-txdb % cd tests
tests % go test ./...
# github.com/DATA-DOG/go-txdb
../db.go:175:2: declared and not used: z
FAIL	github.com/DATA-DOG/go-txdb/tests [build failed]
FAIL

• I verified that running go get github.com/DATA-DOG/go-txdb@d837e05 results in the following go.mod:

module foo

go 1.24.0

require github.com/DATA-DOG/go-txdb v0.2.1-0.20250306135245-d837e056a55c

[flimzy]

Really great solution! Thanks!

[Yiling-J]

@flimzy I think we can release a new version

[flimzy]

@flimzy I think we can release a new version

Done: https://github.com/DATA-DOG/go-txdb/releases/tag/v0.2.1