selftests/ftrace: Add %return suffix tests

Add kprobe %return suffix testcase and syntax error tests
for %return suffix.

Link: https://lkml.kernel.org/r/159972817653.428528.9180599115849301184.stgit@devnote2

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