BTS_Tests: Use Misc_Helpers.f_shutdown consistently

Replace all calls to setverdict(fail) with f_shutdown() since I'd rather
fail and stop early in case we encounter an error.
Only replace setverdict(pass) with f_shutdown() if it is followed by
mtc.stop

Remove internal function f_shutdown and use the one from Misc_Helpers

Change-Id: Ia8b01a1876e969d6f0760ea625e4df83af4f54ca
2 files changed