TST: Adding an autouse test fixture for mpl tests

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

lib/cartopy/tests/mpl/conftest.py

@@ -0,0 +1,21 @@
+# 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.
+import matplotlib.pyplot as plt
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def mpl_test_cleanup(request):
+    """Runs tests in a context manager and closes figures after each test"""
+    try:
+        # Run each test in a context manager so that state does not leak out
+        orig_backend = plt.get_backend()
+        plt.switch_backend('Agg')
+        with plt.rc_context():
+            yield
+    finally:
+        # Closes all open figures and switches backend back to original
+        plt.switch_backend(orig_backend)