Fix unclosed test files

[ConorMacBride]

Fix for

>                   warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))
E                   pytest.PytestUnraisableExceptionWarning: Exception ignored in: <_io.FileIO [closed]>
E                   
E                   Traceback (most recent call last):
E                     File "/home/runner/work/pytest-mpl/pytest-mpl/tests/test_deterministic.py", line 116, in test_config
E                       actual_metadata = Image.open(str(baseline_image)).info
E                   ResourceWarning: unclosed file <_io.BufferedReader name='/tmp/pytest-of-runner/pytest-0/test_config34/baseline/test_mpl.png'>