feat: close coverage gaps in spanner::Client (googleapis/google-cloud-cpp-spanner#1152)
feat: close coverage gaps in spanner::Client (googleapis/google-cloud-cpp-spanner#1152)

Add unit tests for two Client::ProfileSql overloads that were
not tested elsewhere.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp