test: validate SPDX with the JSON schema

[nikpivkin]

Description

This PR adds validation of the SDPX-JSON report using the SPDX JSON schema.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[nikpivkin]

@knqyf263 SPDX does not specify the patch version, so would it be correct to use the version 2.3.1 scheme for validation?

[knqyf263]

@nikpivkin I think so.

[knqyf263]

I'm sure we will forget to bump the version. Can we use spdx.Version somehow as it is used here?

https://github.com/spdx/tools-golang/blob/6a271aaa7370038495ab7acdd0eab38e72666c9c/spdx/v2/v2_3/document.go#L15

I see a problem between v2.3 and v2.3.1. If it is hard to identify the patch version, I think it is ok to use v2.3.

[knqyf263]

@nikpivkin Are you going to open another PR for ValidateDocument? If I understand correctly, we want to call that to validate SPDX documents in our integration tests, right?
https://github.com/spdx/tools-golang/blob/6a271aaa7370038495ab7acdd0eab38e72666c9c/spdxlib/documents.go#L14

[nikpivkin]

@knqyf263 I added document validation in this commit d8dfeb9

[knqyf263]

Oh, I missed it. Thanks.