tests: improve the signal/noise ratio of the test logs by 63%

Before this change, the log of the tests were littered with messages from
Pillow:
    2024-09-10 18:04:55 [   DEBUG] STREAM b'IHDR' 16 13 (PngImagePlugin.py:197)
    2024-09-10 18:04:55 [   DEBUG] STREAM b'sBIT' 41 4 (PngImagePlugin.py:197)
    2024-09-10 18:04:55 [   DEBUG] b'sBIT' 41 4 (unknown) (PngImagePlugin.py:753)
    2024-09-10 18:04:55 [   DEBUG] STREAM b'pHYs' 57 9 (PngImagePlugin.py:197)

After this change, on my machine, the size of the log produced by running
"make test" goes from 1405 to 513 lines, a 63% reduction, that hopefully
increases the signal noise ratio of the test logs.

tests/test_plot/base.py

@@ -44,6 +44,7 @@ def test_run(self) -> None:
     def run(self) -> None:
         """Run the plotting and the image comparison."""
         logging.getLogger("matplotlib.font_manager").disabled = True
+        logging.getLogger("PIL.PngImagePlugin").setLevel(logging.INFO)
         plt.close()
         self.plotting_function.__func__(*self.args)  # type: ignore[attr-defined]
         with TemporaryDirectory() as tmpdir: