bigtable: Extend integration test to verify multiple cell versions more thoroughly.
bigtable: Extend integration test to verify multiple cell versions more thoroughly.

This test is based on the equivalent Java client test.

Also extend the test timeout when talking to prod, since it usually
takes around 1m10s when run on my workstation.

Change-Id: I5fea1d1af07a39cedc565bfefecc2782ebd5a523

Maksadbek/gcloud-golang,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,ptomasroos/gcloud-golang,Maksadbek/gcloud-golang,ptomasroos/gcloud-golang,hsinhoyeh/gcloud-golang,googleapis/google-cloud-go,hsinhoyeh/gcloud-golang,googleapis/google-cloud-go,roth1002/gcloud-golang,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,blowmage/gcloud-golang,blowmage/gcloud-golang,roth1002/gcloud-golang