selftests: fib_tests: Add ipv4 route add append replace tests

Add IPv4 route tests covering add, append and replace permutations.
Assumes the ability to add a basic single path route works; this is
required for example when adding an address to an interface.

$ fib_tests.sh -t ipv4_rt

IPv4 route add / append tests
    TEST: Attempt to add duplicate route - gw                           [ OK ]
    TEST: Attempt to add duplicate route - dev only                     [ OK ]
    TEST: Attempt to add duplicate route - reject route                 [ OK ]
    TEST: Add new nexthop for existing prefix                           [ OK ]
    TEST: Append nexthop to existing route - gw                         [ OK ]
    TEST: Append nexthop to existing route - dev only                   [ OK ]
    TEST: Append nexthop to existing route - reject route               [ OK ]
    TEST: Append nexthop to existing reject route - gw                  [ OK ]
    TEST: Append nexthop to existing reject route - dev only           ...