TST: Using the matplotlib autouse testing fixture

This takes care of all the cleanup of figures and state
for each test.

lib/cartopy/tests/mpl/conftest.py

@@ -0,0 +1,9 @@
+# Copyright Cartopy Contributors
+#
+# This file is part of Cartopy and is released under the LGPL license.
+# See COPYING and COPYING.LESSER in the root of the repository for full
+# licensing details.
+
+# This is a matplotlib autouse fixture that will close all plots
+# and reset any parameters that may have been modified.
+from matplotlib.testing.conftest import mpl_test_settings  # noqa: F401