Python: Add unit test for versionless shim.

[tswast]

We're getting coverage errors due to the google.cloud.bigquery_datatransfer module never being imported in the unit tests.

googleapis/google-cloud-python#8630 (comment)

We should have an auto-generated unit test similar to the one Tres suggests.

[software-dov]

Ported from monolith, not clear it's relevant to the microgenerator. Closing with option to reopen.