kokoro: enable bigquerydatatransfer tests
kokoro: enable bigquerydatatransfer tests

I changed the test to check basic output, rather than project-specific
data sources. So, it's not quite an integration test, but it ensures
there are no errors in the sample.

Part of #251.

GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples,GoogleCloudPlatform/ruby-docs-samples