selftests: fib_tests: Add test cases for interaction with mangling

Test that packets are correctly routed when netfilter mangling rules are
present.

Without previous patch:

 # ./fib_tests.sh -t ipv4_mangle

 IPv4 mangling tests
     TEST:     Connection with correct parameters                        [ OK ]
     TEST:     Connection with incorrect parameters                      [ OK ]
     TEST:     Connection with correct parameters - mangling             [FAIL]
     TEST:     Connection with correct parameters - no mangling          [ OK ]
     TEST:     Connection check - server side                            [FAIL]

 Tests passed:   3
 Tests failed:   2

 # ./fib_tests.sh -t ipv6_mangle

 IPv6 mangling tests
     TEST:     Connection with correct parameters                        [ OK ]
     TEST:     Connection with incorrect parameters                      [ OK ]
     TEST:     Connection with correct parameters - mangling             [FAIL]
     TEST:     Connection with correct parameters - no mangling          [ OK ]
     TEST:     Connection check - server side                            [FAIL]

 Tests passed:   3
 Tests failed:   2

With previous patch:

 # ./fib_tests.sh -t ipv4_mangle

 IPv4 mangling tests
     TEST:     Connection with correct parameters                        [ OK ]
     TEST:     Connection with incorrect parameters                      [ OK ]
     TEST:     Connection with correct parameters - mangling             [ OK ]
     TEST:     Connection with correct parameters - no mangling          [ OK ]
     TEST:     Connection check - server side                            [ OK ]

 Tests passed:   5
 Tests failed:   0

 # ./fib_tests.sh -t ipv6_mangle

 IPv6 mangling tests
     TEST:     Connection with correct parameters                        [ OK ]
     TEST:     Connection with incorrect parameters                      [ OK ]
     TEST:     Connection with correct parameters - mangling             [ OK ]
     TEST:     Connection with correct parameters - no mangling          [ OK ]
     TEST:     Connection check - server side                            [ OK ]

 Tests passed:   5
 Tests failed:   0

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>