selftests/ftrace: Limit length in subsystem-enable tests

While sched* events being traced and sched* events continuously happen,
"[xx] event tracing - enable/disable with subsystem level files" would
not stop as on some slower systems it seems to take forever.
Select the first 100 lines of output would be enough to judge whether
there are more than 3 types of sched events.

Fixes: 815b18ea

 ("ftracetest: Add basic event tracing test cases")
Cc: stable@vger.kernel.org
Signed-off-by: Yuanhe Shu <xiangzao@linux.alibaba.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>