[VAMOS] BTS: call f_shutdown() in f_TC_vamos_exec_async()

Sometimes, VAMOS related test cases fail due to a DTE:

  Dynamic test case error: Sending data on the connection of port
                           CCHAN_PT to 1:RSL_CCHAN failed. (Broken pipe)

Call f_shutdown() to ensure that all components are properly
terminated, so there will be no race conditions like this.

Change-Id: I690412a29b24571109415d32b09543de459076d1
Related: SYS#4895

bts/BTS_Tests_VAMOS.ttcn

@@ -182,6 +182,8 @@ runs on test_CT {
 			vc_conn[ch].done;
 		}
 	}
+
+	Misc_Helpers.f_shutdown(__BFILE__, __LINE__);
 }
 
 private function f_TC_vamos_chan_act(charstring id)