trunks: stop background thread in transceiver unit test

If background thread is still running when background transceiver
is destroyed, it is possible the thread is still holding some
weak pointers to the transceiver. If so, the transceiver
destructor will fail in DCHECK() when the weak pointer factory
will attempt to destruct those remaining pointers.

BUG=chromium:684141
TEST=For a board with tpm2, add "sleep(3)" to the end of
     AssignAndSignal() in background_command_transceiver.cc and run
     P2_TEST_FILTER="BackgroundTransceiverTest.Synchronous" \
     cros_workon_make --board=$BOARD --test trunks

Change-Id: I42c6e373189a2ad4640ef0e7518acfbc6046c383
Reviewed-on: https://chromium-review.googlesource.com/431503
Commit-Ready: Andrey Pronin <apronin@chromium.org>
Tested-by: Andrey Pronin <apronin@chromium.org>
Reviewed-by: Darren Krahn <dkrahn@chromium.org>
1 file changed