bigtable/bttest: Allow more time for concurrent mod test.
bigtable/bttest: Allow more time for concurrent mod test.

Change-Id: I6fd9c2fd23861f78bb161b6e2749b69fe1abfaa8
Reviewed-on: https://code-review.googlesource.com/8450
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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