Move all tests to ./tests/ with new Go module to isolate testing dependencies

[endigma]

Summary

• Moves tests into tests/ package, which is its own go module, to isolate testing dependencies from the main package
  • This makes it an easier decision to include litter in independent, downstream testing packages similar to testify, which was previously an indirect dependency of litter itself
  • It can confuse users when they install a testify alternative and it pulls in testify indirectly
  • This also caused litter to indirectly import its own alternative, spew, via testify
  • Also allows tests to freely import essentially any 3rd party package to add tests when an issue is resolved to prevent regressions
• Uses assert.Equal instead of the diff shellout, which may be uglier but is more reliable, was making a fuss about trailing newlines or something I didn't want to debug

[endigma]

@atombender what does this need to be merged? should I just make a fork?

[atombender]

Hi, sorry about this, I plain forgot about it. I'll look now.

[atombender]

Sorry about the delay. Appreciate the improvements. If you have more work you want to get committed, I'm going to try prioritizing reviews!