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>