Low: resources: clean serialized file on SIGTERM in Dummy

Otherwise it could give a false probe error at next start,
confusing whatever else is being tested with a dummy resource.
Unfortunately this doesn't help if an in-flight monitor gets
cancelled with a SIGKILL, but there's no obvious solution there.

extra/resources/Dummy

@@ -114,6 +114,10 @@ END
 trap sigterm_handler TERM
 sigterm_handler() {
     ocf_log info "They use TERM to bring us down. No such luck."
+
+    # Since we're likely going to get KILLed, clean up any monitor
+    # serialization in progress, so the next probe doesn't return an error.
+    rm -f "${VERIFY_SERIALIZED_FILE}"
     return
 }
 
@@ -171,6 +175,7 @@ dummy_monitor() {
             # two monitor ops have occurred at the same time.
             # This verifies a condition in pacemaker-execd regression tests.
             ocf_log err "$VERIFY_SERIALIZED_FILE exists already"
+            ocf_exit_reason "alternate universe collision"
             return $OCF_ERR_GENERIC
         fi