Added MediaProvider tests (#386)

* Media provider tests

* Moved classes to separate files

* Moved image file

* Removed unused image file

* Rebase and refactor.

Replace removed error type.
Refactor XYZMock to MockXYZ.

Co-authored-by: Doug <douglase@element.io>

NSE/Sources/NotificationServiceExtension.swift

@@ -130,7 +130,7 @@ class NotificationServiceExtension: UNNotificationServiceExtension {
 
         return MediaProvider(mediaProxy: MediaProxy(client: client),
                              imageCache: .onlyOnDisk,
-                             fileCache: .default,
+                             fileCache: FileCache.default,
                              backgroundTaskService: nil)
     }