BTS_Tests: Call mtc.stop in f_shutdown() to fix sporadic test failures

mtc.stop cleanly shuts down a testcase. This avoids errors influencing
the result after the test completed (such as ports being shutdown with
messages arriving on them causing a dynamic test case error).

Change-Id: I0f6e10006f46db0d68796c7082497af90f74cd07
1 file changed