selftests/bpf: test_tunnel: Move gre tunnel test to test_progs

gre tunnels are tested in the test_tunnel.sh but not in the test_progs
framework.

Add a new test in test_progs to test gre tunnels. It uses the same
network topology and the same BPF programs than the script.
Remove test_gre() and test_gre_no_tunnel_key() from the script.

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250303-tunnels-v2-3-8329f38f0678@bootlin.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>