TST: a tiny bit more integration testing in pandas for gbq

[jreback]

after #15484

just need to have a full on write-table (which we do), and read (which we do), but need to do a proper comparison.

[jreback]

cc @parthea

[alimcmaster1]

Closing in favour of testing for this moving to pandas-gbq ref: googleapis/python-bigquery-pandas#379

#43231