test-dhcp: ip route deprecation warning

[Luap99]

#  ip netns exec fFTG9ee6kr ip add del 10.169.119.1 dev br0
# Warning: Executing wildcard deletion to stay compatible with old scripts.
#          Explicitly specify the prefix length (10.169.119.1/32) to avoid this warning.
#          This special behaviour is likely to disappear in further releases,
#          fix your scripts!

Seen in the dhcp tests, likely need to fix the deletion there