ftrace/selftest: Test if set_event/ftrace_pid exists before writing

While testing on a very old kernel (3.5), the tests failed because the write
to set_event_pid in the setup code, did not exist. The tests themselves
could pass, but the setup failed causing an error.

Other files test for existance before writing to them. Do the same for
set_event_pid and set_ftrace_pid.

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>