bigquery: refactor integration tests
bigquery: refactor integration tests

Move initialization of integration tests into a separate function, so we
can split up the monolithic integration test into separate tests.

Also, generate unique table names to minimize the chance that
simultanously running tests will interfere with each other.

Change-Id: I060bd9b77bc37ce7b44735e37c449385dd494a21
Reviewed-on: https://code-review.googlesource.com/8998
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

michael-grosshans/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go