firestore: block when dialing mock server
firestore: block when dialing mock server

We've seen the error

 rpc error: code = Unavailable desc = grpc: the connection is unavailable

when running unit tests on Travis. Hoping this will help.

Change-Id: I67596ca7c3e5efa4600754510acca0e7c824aa16
Reviewed-on: https://code-review.googlesource.com/17411
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