refactor(test): use more specific checks in `check-tsconfig` spec

[agilgur5]

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Summary

Be more precise with the checks in check-tsconfig.spec.ts and reorder things a bit

• check-tsconfig.spec.ts should now match the structure of parse-tsconfig.spec.ts from #397
• Follow-up to #321

Details

• toBeFalsy() was a bit imprecise and always felt as such, especially since checkTsConfig returns void

  • so instead check that it doesn't throw, which matches the test intent

• reorder the tests a bit to match existing test style: non-errors first, then errors

  • and separating into two different test blocks parallelizes them as well

• also add an ES2020 check as well

  • follow-up to eb1dd17babde0b22e9540b12e671eb56d9d6bce0 and #376