selftests/ftrace: Convert check_filter_file() with requires list

Since check_filter_file() is basically checking the filter
tracefs file, we can convert it into requires list.

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