selftests: net: use slowwait to stabilize vrf_route_leaking test

selftests: net: use slowwait to stabilize vrf_route_leaking test

The vrf_route_leaking test occasionally fails due to connectivity issues
in our testing environment. A sample failure message shows that the ping
check fails intermittently

  PING 2001:db8:16:2::2 (2001:db8:16:2::2) 56 data bytes

  --- 2001:db8:16:2::2 ping statistics ---
  1 packets transmitted, 0 received, 100% packet loss, time 0ms

  TEST: Basic IPv6 connectivity                                       [FAIL]

This is likely due to insufficient wait time on slower machines. To address
this, switch to using slowwait, which provides a longer and more reliable
wait for setup completion.

Before this change, the test failed 3 out of 10 times. After applying this
fix, the test was run 30 times without any failure.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250617105101.433718-2-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>