selftests/ftrace: Do not to use absolute debugfs path

Use relative path to trigger file instead of absolute debugfs path,
because if the user uses tracefs instead of debugfs, it can be
mounted at /sys/kernel/tracing.
Anyway, since the ftracetest is designed to be run at the tracing
directory, user doesn't need to use absolute path.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>