fixup! task(backup_tests): Move mock helpers into MatrixMockServer

Disable use of .encryption when e2ee is disabled (in doctest)

crates/matrix-sdk/src/test_utils/mocks.rs

@@ -511,10 +511,13 @@ impl MatrixMockServer {
     ///
     /// mock_server.mock_room_keys_version().exists().expect(1).mount().await;
     ///
-    /// let exists =
-    ///     client.encryption().backups().exists_on_server().await.unwrap();
+    /// # #[cfg(feature = "e2e-encryption")]
+    /// # {
+    ///     let exists =
+    ///         client.encryption().backups().exists_on_server().await.unwrap();
     ///
-    /// assert!(exists);
+    ///     assert!(exists);
+    /// # }
     /// # });
     /// ```
     pub fn mock_room_keys_version(&self) -> MockEndpoint<'_, RoomKeysVersionEndpoint> {