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
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index b77a70a..dbb8313 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -341,10 +341,8 @@
 }
 
 function f_shutdown() runs on test_CT {
-	/* shut down all "externally interfaced" components first to avoid unclean shutdown */
-	vc_IPA.stop;
-	vc_RSL.stop;
-	f_ipa_ctrl_stop();
+	/* mtc.stop cleanly stops testcase execution to avoid unclean shutdown */
+	mtc.stop;
 }
 
 /* Attach L1CTL to master test_CT (classic tests, non-handler mode) */