selftests: net: use slowwait to make sure IPv6 setup finished

selftests: net: use slowwait to make sure IPv6 setup finished

Sometimes the vxlan vnifiltering test failed on slow machines due to
network setup not finished. e.g.

  TEST: VM connectivity over vnifiltering vxlan (ipv4 default rdst)   [ OK ]
  TEST: VM connectivity over vnifiltering vxlan (ipv6 default rdst)   [FAIL]

Let's use slowwait to make sure the connection is finished.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250617105101.433718-3-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>