Gold test for reporting

[bobhancockg]

This commit introduces a test suite for the parallel_report_download.py script.

The tests cover the following:

• Correct generation of inputs for report downloads.
• Successful and failed report requests, including the retry mechanism for API errors.
• The main function's logic for handling parallel processing (via mocked multiprocessing) and aggregation of results.

The tests are located in examples/reporting/tests/ and utilize mocking to simulate Google Ads API interactions, ensuring they run without actual API calls and are specific to API version v19, as used by the script.