Tools: runfliptest.sh: Resolve undefined name

Fixes: #6839 (partial fix)

Undefined names can raise [NameError](https://docs.python.org/2/library/exceptions.html#exceptions.NameError) at runtime.

Tools/scripts/runfliptest.sh

@@ -9,7 +9,7 @@ def wait_heartbeat(mav, timeout=10):
     while time.time() < start_time+timeout:
         if mav.recv_match(type='HEARTBEAT', blocking=True, timeout=0.5) is not None:
             return
-    failure("Failed to get heartbeat")    
+    raise Exception("Failed to get heartbeat")
 
 def wait_mode(mav, modes, timeout=10):
     '''wait for one of a set of flight modes'''