selftests/ftrace: Add "requires:" list support

Introduce "requires:" list to check required ftrace interface
for each test. This will simplify the interface checking code
and unify the error message. Another good point is, it can
skip the ftrace initializing.

Note that this requires list must be written as a shell
comment.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>