BidirectionalStreamTest: don't cancel and block in teardown

Calling callback.blockForDone() on a non-started stream's callback will
block until it timeouts (12s). Currently, only a single test within
BidirectionalStreamTest actually starts the stream. This means that
every other tests is calling callback.blockForDone() during teardown
for no reason at all and they will just get stuck for 12s.

Change-Id: Ia40cbc371b4e2450dcd92a3ad429ff4351931a21