Add mock

so test passes again

test/test-utils/test-utils.ts

@@ -281,6 +281,7 @@ export function createTestClient(): MatrixClient {
         getLocalAliases: jest.fn().mockReturnValue([]),
         uploadDeviceSigningKeys: jest.fn(),
         isKeyBackupKeyStored: jest.fn().mockResolvedValue(null),
+        getKeyBackupVersion: jest.fn(),
     } as unknown as MatrixClient;
 
     client.reEmitter = new ReEmitter(client);