selftests: ftrace: Add a testcase for function PID filter

selftests: ftrace: Add a testcase for function PID filter

Like event pid filtering test, add function pid filtering test with the
new "function-fork" option.  It also tests it on an instance directory
so that it can verify the bug related pid filtering on instances.

Link: http://lkml.kernel.org/r/20170417024430.21194-5-namhyung@kernel.org
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>