showcase unit tests missing for rest transport

[arithmetic1728]

It seems circleci currently only generates and runs the showcase unit tests for grpc transport. We need some unit tests for rest transport with showcase protos.

[yon-mg]

This is intended. Currently the only way to generate the rest transport is including "rest" in the "transport" option of the python_gapic_out flag (e.g. --python_gapic_out="transport=grpc+rest". This was decided instead of maintaining a separate long running branch here, #688 (comment).

Since all features have not yet been implemented, Showcase unit tests on rest transport are guaranteed to fail. This in combination with the above led to the choice of not including rest generation for CircleCI until rest transport features are complete.

[arithmetic1728]

@yon-mg I see. Thank you for the details! Let's mark it p3.

[software-dov]

Tests have been added for both echo and compliance.