vision: parallelize tests
vision: parallelize tests

Speed up tests by running them in parallel.

Change-Id: Ife0e38636bc9bd2f7bc976eefdb09141cac7a7f2
Reviewed-on: https://code-review.googlesource.com/11711
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go