Add additional showcase tests related to automatically populating fields

[parthea]

In PR googleapis/gapic-generator-python#1985, support was added to automatically populate fields UUID4 fields according to https://google.aip.dev/client-libraries/4235.

This issue tracks adding additional showcase tests once the following feature has been added to showcase:

• Add fail_n_times field to add ability to configure a request to fail x number of times before receiving a valid response gapic-showcase#1472

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.

[daniel-sanche]

This issue is being closed as part of a bulk review (Filter #1). Please reopen if you believe this is still relevant and requires action.