selftests/ftrace: Clean up triggers after setting them

The triggers set in trigger-onchange-action-hist.tc and
trigger-snapshot-action-hist.tc are not cleaned up at the end. These tests
can also be done in instances and without cleaning up the triggers, the
instances can not be removed as they are still "busy".

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-onchange-action-hist.tc

···
     fail "Failed to create onchange action inter-event histogram"
 fi
 
+echo '!hist:keys=comm:newprio=prio:onchange($newprio).save(comm,prio) if comm=="ping"' >> events/sched/sched_waking/trigger
+
 exit 0

tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-snapshot-action-hist.tc

···
     fail "Failed to create snapshot action inter-event histogram"
 fi
 
+echo '!hist:keys=comm:newprio=prio:onchange($newprio).save(comm,prio):onchange($newprio).snapshot() if comm=="ping"' >> events/sched/sched_waking/trigger
+
 exit 0