selftests/bpf: test_tunnel: Add generic_attach* helpers

A fair amount of code duplication is present among tests to attach BPF
programs.

Create generic_attach* helpers that attach BPF programs to a given
interface.
Use ASSERT_OK_FD() instead of ASSERT_GE() to check fd's validity.
Use these helpers in all the available tests.

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-v2-1-8329f38f0678@bootlin.com


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