test: add `parse-tsconfig` spec

[agilgur5]

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Summary

Adds unit tests for a few scenarios of parse-tsconfig.ts

• Follow-up to #321 and https://github.com/ezolenko/rollup-plugin-typescript2/pull/371#issuecomment-1174339120

Details

• test passing tsconfig, buggy tsconfig, non-existent tsconfig, and not a tsconfig

• clean: "failed to read" error will never happen as we already checked for existence of the file earlier

  • so remove the undefined check and instead use a non-null assertion (plus a comment explaining it)

• refactor: move the integration test for tsconfig error into this unit test instead

  • faster / more efficient / more precise

• refactor: split out a makeOptions func that creates default plugin options to use in tests

  • similar to makeStubbedContext

Future Work

• Could also test that merging tsconfigs works correctly, not just that it doesn't throw
  • Would be particularly useful for when #86 is implemented as that would no longer be a straight lodash deep merge anymore