selftests/ftrace: Add '!event' synthetic event syntax check

Add a check confirming that '!event' alone will remove a synthetic
event.

Link: https://lkml.kernel.org/r/1dff3f03d18542cece08c10d6323d8a8dba11e42.1612208610.git.zanussi@kernel.org

Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-synthetic-event-syntax.tc

@@ -32,6 +32,10 @@ grep "myevent[[:space:]]u64 var1" synthetic_events
 # it is not possible to add same name event
 ! echo "myevent u64 var2" >> synthetic_events
 
+# make sure !synthetic event doesn't require a field
+echo "!myevent" >> synthetic_events
+echo "myevent u64 var1" >> synthetic_events
+
 # Non-append open will cleanup all events and add new one
 echo "myevent u64 var2" > synthetic_events