selftests/ftrace: Support ":tracer" suffix for requires

Add ":tracer" suffix support for the requires list, so that
the testcase can list up the required tracer (e.g. function)
to the requires list.

For example, if the testcase requires function_graph tracer,
it can write requires list as below instead of checking
available_tracers.

# requires: function_graph:tracer

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