selftests/bpf: Migrate test_xdp_meta.sh into xdp_context_test_run.c

test_xdp_meta.sh can't be used by the BPF CI.

Migrate test_xdp_meta.sh in a new test case in xdp_context_test_run.c.
It uses the same BPF programs located in progs/test_xdp_meta.c and the
same network topology.
Remove test_xdp_meta.sh and its Makefile entry.

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20241213-xdp_meta-v2-2-634582725b90@bootlin.com