selftests/ftrace: Improve kretprobe testcase to check log data

Improve kretprobe testcase to check the log data correctness
and ensure the event definition is corrctly including
argument definition.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>