More testing by registering mplcairo as default backend for all formats.

Author: anntzer

run-mpl-test-suite.py

@@ -76,6 +76,8 @@ def _raise_on_image_difference(expected, actual, tol):
         mplcairo.base.GraphicsContextRendererCairo
     mpl.backends.backend_agg = \
         sys.modules["matplotlib.backends.backend_agg"] = mplcairo.base
+    mpl.backend_bases._default_backends = {
+        k: "mplcairo.base" for k in mpl.backend_bases._default_backends}
 
     with warnings.catch_warnings(record=True):  # mpl 3.0
         mpl.use("agg", force=True)