selftests/ftrace: Add BTF fields access testcases

Add test cases for accessing the data structure fields using BTF info.
This includes the field access from parameters and retval, and accessing
string information.

Link: https://lore.kernel.org/all/169272161265.160970.14048619786574971276.stgit@devnote2/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>