selftests/bpf: test_xdp_veth: Use unique names

selftests/bpf: test_xdp_veth: Use unique names

The network namespaces and the veth used by the tests have hardcoded
names that can conflict with other tests during parallel runs.

Use the append_tid() helper to ensure the uniqueness of these names.
Use the static network configuration table as a template on which
thread IDs are appended in each test.
Set a fixed size to remote_addr field so the struct veth_configuration
can also have a fixed size.

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/20250131-redirect-multi-v4-9-970b33678512@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>