selftests/ftrace: Have pid filter test use instance flag

While running the ftracetests, the pid filter test failed because the
instance "foo" existed, and it was using it to rerun the test under a
instance named foo. The collision caused the test to fail as the mkdir
failed as the name already existed.

As of commit b5b77be8

 ("selftests: ftrace: Allow some tests to be run
in a tracing instance") all a selftest needs to do to be tested in an
instance is to set the "instance" flag. There's no reason a selftest needs
to create an instance to run its test in an instance directly.

Remove the open coded testing in an instance for the pid filter test and
have it set the "instance" flag instead.

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