selftests/ftrace: Add funcgraph-retval test case

Add a test case for the funcgraph-retval and funcgraph-retval-hex
trace options.

Link: https://lkml.kernel.org/r/9fedbd25e63f012cade5dad13be21225fec2fb5d.1680954589.git.pengdonglin@sangfor.com.cn

Signed-off-by: Donglin Peng <pengdonglin@sangfor.com.cn>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>