tools/bpf: Add bpf_map_lookup_elem selftest for xskmap

Check that bpf_map_lookup_elem lookup and structure
access operats correctly.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>