pubsub: fix TestStreamingPullAck race
pubsub: fix TestStreamingPullAck race

re: https://sponge.corp.google.com/target?id=98425512-e687-4419-a2c4-5296cb727ee1&target=cloud-go-libraries%2Fpresubmit%2Fgo111&searchFor=

TestStreamingPullAck has a data race because it relies on calling
spc.CloseSend concurrently with spc.Send, which is an unsafe
sequence of operations [1]. This CL switches the CloseSend out
for a context cancellation, which accomplishes the same thing
but is safe to use.

It also reduces the time to wait for dupes from 3s to 2s, to
speed up the test a bit.

1: grpc-go documentation being amended to describe this, see
here: https://github.com/grpc/grpc-go/pull/2418

Change-Id: I254091a664a7d03852568473fa2038751fcc2718
Reviewed-on: https://code-review.googlesource.com/c/34890
Reviewed-by: Eno Compton <ad9228447a2ef39a5712f4b80b45071fa843fa0d@google.com>

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