Increase code coverage in generated client libraries

[askmeegs]

googleapis/java-cloudbuild#343

see: codecov/patch — 77.14% of diff hit (target 81.69%)

[miraleung]

Summarizing an offline discussion: This seems to be caused by the removal of @BetaApi annotations, which probably were previously skipped by code coverage. There doesn't seem to be an effective delta, but we should increase coverage in a future microgenerator release.

[blakeli0]

We are working on new test strategy for generated client libraries, the goal may not be improving the code coverage number.