selftests: net: make busywait timeout clock portable

selftests: net: make busywait timeout clock portable

loopy_wait() expects millisecond timestamps. However, Ubuntu Resolute
can use uutils date, where `date -u +%s%3N` returns seconds plus full
nanoseconds instead of a 3-digit millisecond field. This makes
busywait expire too early and can make vlan_bridge_binding.sh read a
stale operstate.

Fixes: 25ae948b4478 ("selftests/net: add lib.sh")
Cc: stable@vger.kernel.org # 6.8+
Link: https://github.com/uutils/coreutils/issues/11658
Signed-off-by: Nirmoy Das <nirmoyd@nvidia.com>
Link: https://patch.msgid.link/20260630165157.3814871-1-nirmoyd@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>