TST : move test_axes_grid1 to mpl_toolkits/tests

 - moved test file
 - updated whitelisted testing modules

closes matplotlib#3822

Author: tacaswell

lib/matplotlib/__init__.py

@@ -1377,7 +1377,6 @@ def tk_window_focus():
     'matplotlib.tests.test_arrow_patches',
     'matplotlib.tests.test_artist',
     'matplotlib.tests.test_axes',
-    'matplotlib.tests.test_axes_grid1',
     'matplotlib.tests.test_backend_bases',
     'matplotlib.tests.test_backend_pdf',
     'matplotlib.tests.test_backend_pgf',
@@ -1423,6 +1422,7 @@ def tk_window_focus():
     'matplotlib.tests.test_transforms',
     'matplotlib.tests.test_triangulation',
     'mpl_toolkits.tests.test_mplot3d',
+    'mpl_toolkits.tests.test_axes_grid1',
     ]