selftests: test_bridge_neigh_suppress.sh: Fix failures due to duplicate MAC

selftests: test_bridge_neigh_suppress.sh: Fix failures due to duplicate MAC

When creating the topology for the test, three veth pairs are created in
the initial network namespace before being moved to one of the network
namespaces created by the test.

On systems where systemd-udev uses MACAddressPolicy=persistent (default
since systemd version 242), this will result in some net devices having
the same MAC address since they were created with the same name in the
initial network namespace. In turn, this leads to arping / ndisc6
failing since packets are dropped by the bridge's loopback filter.

Fix by creating each net device in the correct network namespace instead
of moving it there from the initial network namespace.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20240426074015.251854d4@kernel.org/
Fixes: 7648ac72dcd7 ("selftests: net: Add bridge neighbor suppression test")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20240507113033.1732534-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>