selftests: bonding: add delay before each xvlan_over_bond connectivity check

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 <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20251114082014.750edfad@kernel.org
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20251127143310.47740-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>