selftests/ftrace: Fix ftrace test cases to check unsupported

Since dynamic function tracer can be disabled, set_ftrace_filter
can be disappeared. Test cases which depends on it, must check
whether the set_ftrace_filter exists or not before testing
and if not, return as unsupported.

Also, if the function tracer itself is disabled, we can not
set "function" to current_tracer. Test cases must check it
before testing, and return as unsupported.

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