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/assets/test.conf

@@ -0,0 +1,8 @@
+[sync_app]
+host = 127.0.0.1
+port = 27701
+data_root = ./collections
+base_url = /sync/
+base_media_url = /msync/
+auth_db_path = ./auth.db
+session_db_path = ./session.db