Generator itself should run generated unit tests on generated clients

[vchudnov-g]

We run generated unit tests in the individual GAPIC repos, and that is as it should be.

However, we should also run generated unit tests upstream in the process, as part of the generator CI. These tests should ideally encompass the full matrix:

{regular templates, Ads templates}
   ×
{transport=rest, transport=grpc}
  ×
(for rest) {numericEnums on, numericEnums off}

though perhaps some combinations are easily doable at the moment and may need to be deferred. The most obvious direction here is to do this using the Showcase protos as much as possible.

(This issue created as a refinement of #1404)