selftests/ftrace: Add test to test new set_ftrace_notrace_pid file

A new file was added to the tracing directory that will allow a user to
place a PID into it and the task associated to that PID will not be traced
by the function tracer. If the function-fork option is enabled, then neither
will the children of that task be traced by the function tracer.

Cc: linux-kselftest@vger.kernel.org
Cc: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>