[BUG]: Test file checker doesn't catch existing tests

[BenHenning]

Describe the bug

If an existing test is exempted with type test_file_not_required then TestFileCheck will happily ignore it even though it exists.

Steps To Reproduce

Add an exemption for an existing test to test_file_exemptions.textproto, e.g.:

test_file_exemption {
  exempted_file_path: "utility/src/main/java/org/oppia/android/util/math/MathExpressionParser.kt"
  test_file_not_required: true
}

Then run the check:

bazel run //scripts:test_file_check -- $(pwd)

MathExpressionParserTest exists yet the above will still pass.

Expected Behavior

TestFileCheck should fail if an existing test is marked as not needed because it obviously exists.

Screenshots/Videos

No response

What device/emulator are you using?

No response

Which Android version is your device/emulator running?

No response

Which version of the Oppia Android app are you using?

No response

Additional Context

No response