fix(jasmine-this): prevent TS annotations in non-TS files

[puglyfe]

Closes #623

This adds an extra check to the jasmine-this transform (which is also used in the mocha transform) to ensure that the source file is actually a TS file rather than just using a TS parser before adding any TS annotations. The tests were previously confirming the existence of this bug, as the filename was hardcoded to test.js, but the test asserted that a TS annotation was added.

To test this, I made a backwards-compatible change to the runPlugin test util to allow setting the source file name.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.73%. Comparing base (a94e2b5) to head (16c32f9).
Report is 81 commits behind head on main.

Files with missing lines	Patch %	Lines
src/utils/test-helpers.ts	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #624      +/-   ##
==========================================
+ Coverage   92.38%   92.73%   +0.35%     
==========================================
  Files          26       27       +1     
  Lines        1944     2093     +149     
  Branches      405      443      +38     
==========================================
+ Hits         1796     1941     +145     
- Misses        102      103       +1     
- Partials       46       49       +3     

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[skovhus]

Thanks!