selftests/ftrace: event_triggers: wait longer for test_event_enable

In some platform, the schedule event may came slowly, delay 100ms can't
cover it.

I was notice that on my board which running in low cpu_freq,and this
selftests allways gose fail.

So maybe we can check more times here to wait longer.

Fixes: 43bb45da82 ("selftests: ftrace: Add a selftest to test event enable/disable func trigger")
Signed-off-by: Yipeng Zou <zouyipeng@huawei.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/ftrace/test.d/ftrace/func_event_triggers.tc

@@ -38,11 +38,18 @@ cnt_trace() {
 
 test_event_enabled() {
     val=$1
+    check_times=10		# wait for 10 * SLEEP_TIME at most
 
-    e=`cat $EVENT_ENABLE`
-    if [ "$e" != $val ]; then
-	fail "Expected $val but found $e"
-    fi
+    while [ $check_times -ne 0 ]; do
+	e=`cat $EVENT_ENABLE`
+	if [ "$e" == $val ]; then
+	    return 0
+	fi
+	sleep $SLEEP_TIME
+	check_times=$((check_times - 1))
+    done
+
+    fail "Expected $val but found $e"
 }
 
 run_enable_disable() {