selftests: bonding: add delay before each xvlan_over_bond connectivity check

Jakub reported increased flakiness in bond_macvlan_ipvlan.sh on regular
kernel, while the tests consistently pass on a debug kernel. This suggests
a timing-sensitive issue.

To mitigate this, introduce a short sleep before each xvlan_over_bond
connectivity check. The delay helps ensure neighbor and route cache
have fully converged before verifying connectivity.

The sleep interval is kept minimal since check_connection() is invoked
nearly 100 times during the test.

Fixes: 246af950b940 ("selftests: bonding: add macvlan over bond testing")
Reported-by: Jakub Kicinski <[e-mail]>
Closes: https://lore.kernel.org/netdev/[e-mail]
Signed-off-by: Hangbin Liu <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/drivers/net/bonding/bond_macvlan_ipvlan.sh

···
 	local message=${3}
 	RET=0
 
+	sleep 0.25
 	ip netns exec ${ns} ping ${target} -c 4 -i 0.1 &>/dev/null
 	check_err $? "ping failed"
 	log_test "${bond_mode}/${xvlan_type}_${xvlan_mode}: ${message}"