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: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>