tests: net: misc: Use different subnets on test interfaces

The test interfaces registered addresses within the same subnet. As now
only the subnet part of the address is verified when choosing source
address or interface, it was not deterministic which interface would be
selected. Therefore, make the two interfaces use two different subnets
so that tests that verified which address/interface was used work
reliably as earlier.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>