test: Add unit tests for the App class and improve Runner initialization tests

-   Added `tests/unittests/apps/test_apps.py` with basic tests for `App` initialization.
-   Modified `tests/unittests/test_runners.py` to include a test that verifies `Runner` raises a `ValueError` when both `app` and `app_name` are provided during initialization.

PiperOrigin-RevId: 803556826

tests/unittests/apps/__init__.py

@@ -0,0 +1,13 @@
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.