selftests/bpf: test_xdp_veth: Split network configuration

selftests/bpf: test_xdp_veth: Split network configuration

configure_network() does two things : it first creates the network
topology and then configures the BPF maps to fit the test needs. This
isn't convenient if we want to re-use the same network topology for
different test cases.

Rename configure_network() create_network().
Move the BPF configuration to the test itself.
Split the test description in two parts, first the description of the
network topology, then the description of the test case.
Remove the veth indexes from the ASCII art as dynamic ones are used

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/20250131-redirect-multi-v4-5-970b33678512@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>