selftests: forwarding: vxlan_bridge_1d: Check aging while forwarding

Extend the VXLAN FDB aging test case to verify that FDB entries are aged
out when they only forward traffic and not refreshed by received
traffic.

The test fails before "vxlan: Age out FDB entries based on 'updated'
time":

 # ./vxlan_bridge_1d.sh
 [...]
 TEST: VXLAN: Ageing of learned FDB entry                            [FAIL]
 [...]
 # echo $?
 1

And passes after it:

 # ./vxlan_bridge_1d.sh
 [...]
 TEST: VXLAN: Ageing of learned FDB entry                            [ OK ]
 [...]
 # echo $?
 0

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20250204145549.1216254-9-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/vxlan_bridge_1d.sh

@@ -740,6 +740,8 @@ test_learning()
 
 	vxlan_flood_test $mac $dst 0 10 0
 
+	# The entry should age out when it only forwards traffic
+	$MZ $h1 -c 50 -d 1sec -p 64 -b $mac -B $dst -t icmp -q &
 	sleep 60
 
 	bridge fdb show brport vx1 | grep $mac | grep -q self