feat(pubsub/pstest): add ability to create a pstest server listening on a specified port

[caod123]

In some of our tests we handle port allocations manually and would prefer to pass in an allocated port for our pstest Server dependency rather than having the pstest.NewServer call allocate an available port.

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[caod123]

@googlebot I signed it!

[hongalex]

LGTM pending tests

[dc185333]

Rebased on master, may need to force-run kokoro again

[hongalex]

Thanks for the fix @dc185333, merged.