selftests/bpf: Cover write access to skb metadata via dynptr

Add tests what exercise writes to skb metadata in two ways:
1. indirectly, using bpf_dynptr_write helper,
2. directly, using a read-write dynptr slice.

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-v7-7-8a39e636e0fb@cloudflare.com