Use svg instead of png for font manager memory leak test

Some versions of Pillow (notably v9.2.0 is what I had installed) trigger a failure on this test that looks the same as if it was a failure on our own part. v9.4.0 of Pillow does not have the same behavior, hence CI does not catch it.

This changes to svg to avoid the external dependency triggering the failure. The fontmanager lookup that we are explicitly trying to test here does still trigger for svg output, but the pillow generated one does not

Author: ksunden

lib/matplotlib/tests/test_font_manager.py

@@ -340,7 +340,7 @@ def inner():
         with BytesIO() as out:
             with warnings.catch_warnings():
                 warnings.filterwarnings('ignore')
-                fig.savefig(out, format='png')
+                fig.savefig(out, format='svg')
 
     inner()