Create validation checks against spdx-testbed

[lumjjb]

Evaluate the golang tools against the conformance suite (https://github.com/spdx/spdx-testbed) to be able to help make improvements to the code and consistency across SPDX tools.

It would be nice to first have a POC to integrate this and then have it be a advisory check on a CI, which can later be switched to enforce once the lilbrary is passing the suite!

[lumjjb]

This issue could help resolve:

• License expression modeling and syntax #158
• Providing more validation of string fields #159

[neilnaveen]

I would like to help with this, how would you want to implement these changes.

cc @lumjjb