test: add print-diagnostics spec

[agilgur5]

NOTE: this is built on top of #404 as it uses the context changes made there. #404 is itself built on top of #397. As such, I've marked this PR as "Draft" until those PRs are merged.
Rebased on top and marked as ready for review.

Summary

Adds unit tests for all scenarios of print-diagnostics.ts

• Follow-up to test: add initial unit test suite #321

Details

• test all the different categories and formattings
• full unit test coverage now! (except for index/tscache, which integration tests cover)
  • (since this is in "Draft" status right now, full coverage does also depend on test: add parse-tsconfig spec #397 being merged)