test(bigtable): increase ClientOptions test coverage (#6945)
test(bigtable): increase ClientOptions test coverage (#6945)

Add missing unit tests for some of the `ClientOptions`. This will give us more confidence in our code when we transition the `ClientOptions` implementation to use `Options`
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp