selftests/bpf: test_xdp_veth: Use a dedicated namespace

selftests/bpf: test_xdp_veth: Use a dedicated namespace

Tests use the root network namespace, so they aren't fully independent
of each other. For instance, the index of the created veth interfaces
is incremented every time a new test is launched.

Wrap the network topology in a network namespace to ensure full
isolation. Use the append_tid() helper to ensure the uniqueness of this
namespace's name during parallel runs.
Remove the use of the append_tid() on the veth names as they now belong
to an already unique namespace.
Simplify cleanup_network() by directly deleting the namespaces

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/20250212-redirect-multi-v5-2-fd0d39fca6e6@bootlin.com