selftests/ftrace: Limit length in subsystem-enable tests

commit 1a4ea83a6e67f1415a1f17c1af5e9c814c882bb5 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>