TEST: simplify GIFTI warning check

[ananyavaidya7]

Summary

Simplifies the warning check in test_parse_dataarrays by replacing manual warning capture and length checking with pytest.warns(UserWarning).

This also removes the now-unused clear_and_catch_warnings import from the test module.

Closes #810.

Testing

• python -m pytest nibabel/gifti/tests/test_parse_gifti_fast.py::test_parse_dataarrays -v
• Result: 1 passed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.48%. Comparing base (4704d90) to head (8366ee8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1529      +/-   ##
==========================================
- Coverage   95.48%   95.48%   -0.01%     
==========================================
  Files         209      209              
  Lines       30050    30047       -3     
  Branches     4494     4494              
==========================================
- Hits        28692    28689       -3     
  Misses        926      926              
  Partials      432      432              

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