selftests/bpf: Pass just bpf_map to xdp_context_test helper

selftests/bpf: Pass just bpf_map to xdp_context_test helper

Prepare for parametrizing the xdp_context tests. The assert_test_result
helper doesn't need the whole skeleton. Pass just what it needs.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Jesse Brandeburg <jbrandeburg@cloudflare.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20250814-skb-metadata-thru-dynptr-v7-4-8a39e636e0fb@cloudflare.com