selftests/bpf: Migrate test_xdp_redirect.sh to xdp_do_redirect.c

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

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

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250110-xdp_redirect-v2-2-b8f3ae53e894@bootlin.com