test: ensure `declarationMap` sources are correct

[agilgur5]

~NOTE: this is built on top of #386 as the testDir changes there are required to have a correct outDir. As such, I've marked this PR as "Draft" until that PR is merged.~ Rebased on top and marked as ready for review.

Summary

Add a test for declarationMap sources to make sure they're correct after #221 / #204

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

Details

• since we set a placeholder outDir initially, TS sets declaration map sources to that placeholder dir
  • then, after #221, we remap that to the correct dir that Rollup outputs to
  • this test checks that the remap happens and is correct

[agilgur5]

CI failed on an integration test suite timeout, but this PR itself shouldn't have increased the time by more than some milliseconds, so think it makes sense to just re-run CI in this case.