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: 43bb45da

 ("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>