TST: clear renderer cache in tests

Only on windows test_metrics_cache2 was starting with 2 renderers
in the cache.

Given that his is only happening on windows (and not osx/linux) on CI going
with the assumption that these is something being held alive longer than we
expect someplace else in the tests.

Author: tacaswell

lib/matplotlib/tests/test_text.py

@@ -918,7 +918,7 @@ def test_metrics_cache2():
         mpl.text._get_text_metrics_function
     ).parameters['_cache'].default
     gc.collect()
-    assert len(renderer_cache) == 0
+    renderer_cache.clear()
 
     def helper():
         fig, ax = plt.subplots()