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>