TestMixin: don't install ".gitkeep" files.

Files called ".gitkeep" are used as placeholders to tell git to create
the directory.  They shouldn't be copied as part of the demo project
content (in fact, a file called ".gitkeep" would not be allowed by the
file upload form.)

For example, the demo active project AGlSZa5VTv5Dg09CSpoh contains no
actual files, but the git repository includes the file
demo-files/media/active-projects/AGlSZa5VTv5Dg09CSpoh/.gitkeep - so in
this case, TestMixin should create an empty project directory.

physionet-django/user/test_views.py

@@ -140,12 +140,14 @@ def setUp(self):
         """
         _force_delete_tree(settings.MEDIA_ROOT)
         shutil.copytree(os.path.abspath(os.path.join(settings.DEMO_FILE_ROOT, 'media')),
-            settings.MEDIA_ROOT)
+                        settings.MEDIA_ROOT,
+                        ignore=shutil.ignore_patterns('.gitkeep'))
 
         self.test_static_root = settings.STATIC_ROOT if settings.STATIC_ROOT else settings.STATICFILES_DIRS[0]
         _force_delete_tree(self.test_static_root)
         shutil.copytree(os.path.abspath(os.path.join(settings.DEMO_FILE_ROOT, 'static')),
-            self.test_static_root)
+                        self.test_static_root,
+                        ignore=shutil.ignore_patterns('.gitkeep'))
 
         if os.path.exists(ORIGINAL_DBCAL_FILE):
             os.symlink(ORIGINAL_DBCAL_FILE, DBCAL_FILE)