Add test suite for Google Ads Account Analyzer Demo and ensure v19 API usage.

This commit introduces a test suite for the `analyzer.py` script located in `examples/google-ads-account-analyzer-demo/`.

Key changes include:
- Created a new `tests` subdirectory within `examples/google-ads-account-analyzer-demo/`.
- Added `test_analyzer.py` containing unit tests for the `account_hierarchy_module` and `get_users_module` functions.
- Implemented comprehensive mocking for the Google Ads API client, services, and API responses to ensure tests are isolated and cover various scenarios:
    - `account_hierarchy_module`: Tested with and without a specific customer ID, simulating different account structures and API responses.
    - `get_users_module`: Tested by mocking API responses for user access information.
- Ensured that the Google Ads API v19 is usable by confirming the current client library version (26.1.0) already includes v19 support, making library updates unnecessary for this specific requirement. The tests are written to be compatible with v19 API structures where applicable.
- Added `__init__.py` to the new test directory.
- All tests pass, verifying the core functionality of the analyzer script and its output formatting.

This commit fixes an `ImportError: attempted relative import beyond top-level package` that occurred when running tests in `examples/google-ads-account-analyzer-demo/tests/test_analyzer.py` using standard test discovery.

The import mechanism for the `analyzer` module within `test_analyzer.py` has been made more robust by dynamically adding the parent directory (`examples/google-ads-account-analyzer-demo`) to `sys.path` before attempting the import. This ensures that the `analyzer` module can be correctly located regardless of how the tests are invoked (e.g., direct execution, `unittest discover`).

The changes ensure that tests can be reliably run from the repository root using commands like `python -m unittest discover -s examples/google-ads-account-analyzer-demo/tests -p test_analyzer.py`.