FindCxxTest: Add CxxTest_FOUND variable and update docs

- This module previously didn't define the CxxTest_FOUND result variable
  consistently if neither Python nor Perl were found.
- Documentation is also updated a bit further to note that Perl-based
  test generator script has been removed in CxxTest version 4.0.
- Internal comments synced.
- Python_FOUND result variable used.

Issue: #27242