selftests/ftrace: Reduce trace buffer checking overhead

Current event/toplevel-enable.tc checking the trace
buffer by dumping all events while recording events.
However, this makes system very busy.

To reduce this overhead comes from reading trace buffer
and recording trace buffer, use head instead of cat
and stop tracing while reading.
Signed-off-by: NMasami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/ftrace/test.d/event/toplevel-enable.tc

@@ -28,7 +28,9 @@ echo '*:*' > set_event
 
 yield
 
-count=`cat trace | grep -v ^# | wc -l`
+echo 0 > tracing_on
+
+count=`head -n 128 trace | grep -v ^# | wc -l`
 if [ $count -eq 0 ]; then
     fail "none of events are recorded"
 fi
@@ -36,10 +38,12 @@ fi
 do_reset
 
 echo 1 > events/enable
+echo 1 > tracing_on
 
 yield
 
-count=`cat trace | grep -v ^# | wc -l`
+echo 0 > tracing_on
+count=`head -n 128 trace | grep -v ^# | wc -l`
 if [ $count -eq 0 ]; then
     fail "none of events are recorded"
 fi