Add more test helpers and integration tests for SyncApp's media sync feature using WebTest's TestApp.

Add test helpers for creating, inspecting and manipulating instances of SyncApp and RestApp.
Add subclasses of Anki's RemoteServer and RemoteMediaServer for communicating with the wrapped SyncApp instance under test.
Add helpers for monkey patching Anki's MediaManager and DB for easier testing.
Add test assets directory.

tests/helpers/file_utils.py

@@ -169,3 +169,19 @@ class FileUtils(object):
         zip_file.close()
 
         return file_buffer.getvalue()
+
+    def get_asset_path(self, relative_file_path):
+        """
+        Retrieves the path of a file for testing from the "assets" directory.
+
+        :param relative_file_path: the name of the file to retrieve, relative
+                                   to the "assets" directory
+        :return: the absolute path to the file in the "assets" directory.
+        """
+
+        join = os.path.join
+
+        script_dir = os.path.dirname(os.path.realpath(__file__))
+        support_dir = join(script_dir, os.pardir, "assets")
+        res = join(support_dir, relative_file_path)
+        return res