selftests/bpf: De-veth-ize the tc_redirect test case

No functional changes to the test case, but just renaming various functions,
variables, etc, to remove veth part of their name for making it more generic
and reusable later on (e.g. for netkit).

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20231114004220.6495-8-daniel@iogearbox.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>