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.

Link: https://github.com/uutils/coreutils/issues/11658
Signed-off-by: Nirmoy Das <nirmoyd@nvidia.com>
Link: https://patch.msgid.link/20260626144902.3214350-1-nirmoyd@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/lib.sh

@@ -70,12 +70,27 @@ ksft_exit_status_merge()
 		$ksft_xfail $ksft_pass $ksft_skip $ksft_fail
 }
 
+timestamp_ms()
+{
+	local now=$(date -u +%s:%N)
+	local seconds=${now%:*}
+	local nanoseconds=${now#*:}
+
+	if [[ $nanoseconds =~ ^[0-9]+$ ]]; then
+		nanoseconds=${nanoseconds:0:9}
+	else
+		nanoseconds=0
+	fi
+
+	echo $((seconds * 1000 + 10#$nanoseconds / 1000000))
+}
+
 loopy_wait()
 {
 	local sleep_cmd=$1; shift
 	local timeout_ms=$1; shift
 
-	local start_time="$(date -u +%s%3N)"
+	local start_time=$(timestamp_ms)
 	while true
 	do
 		local out
@@ -84,7 +99,7 @@ loopy_wait()
 			return 0
 		fi
 
-		local current_time="$(date -u +%s%3N)"
+		local current_time=$(timestamp_ms)
 		if ((current_time - start_time > timeout_ms)); then
 			echo -n "$out"
 			return 1