selftests: net: Add a selftest for externally validated neighbor entries

Add test cases for externally validated neighbor entries, testing both
IPv4 and IPv6. Name the file "test_neigh.sh" so that it could be
possibly extended in the future with more neighbor test cases.

Example output:

 # ./test_neigh.sh
 TEST: IPv4 "extern_valid" flag: Add entry                           [ OK ]
 TEST: IPv4 "extern_valid" flag: Add with an invalid state           [ OK ]
 TEST: IPv4 "extern_valid" flag: Add with "use" flag                 [ OK ]
 TEST: IPv4 "extern_valid" flag: Replace entry                       [ OK ]
 TEST: IPv4 "extern_valid" flag: Replace entry with "managed" flag   [ OK ]
 TEST: IPv4 "extern_valid" flag: Replace with an invalid state       [ OK ]
 TEST: IPv4 "extern_valid" flag: Interface down                      [ OK ]
 TEST: IPv4 "extern_valid" flag: Carrier down                        [ OK ]
 TEST: IPv4 "extern_valid" flag: Transition to "reachable" state     [ OK ]
 TEST: IPv4 "extern_valid" flag: Transition back to "stale" state    [ OK ]
 TEST: IPv4 "extern_valid" flag: Forced garbage collection           [ OK ]
 TEST: IPv4 "extern_valid" flag: Periodic garbage collection         [ OK ]
 TEST: IPv6 "extern_valid" flag: Add entry                           [ OK ]
 TEST: IPv6 "extern_valid" flag: Add with an invalid state           [ OK ]
 TEST: IPv6 "extern_valid" flag: Add with "use" flag                 [ OK ]
 TEST: IPv6 "extern_valid" flag: Replace entry                       [ OK ]
 TEST: IPv6 "extern_valid" flag: Replace entry with "managed" flag   [ OK ]
 TEST: IPv6 "extern_valid" flag: Replace with an invalid state       [ OK ]
 TEST: IPv6 "extern_valid" flag: Interface down                      [ OK ]
 TEST: IPv6 "extern_valid" flag: Carrier down                        [ OK ]
 TEST: IPv6 "extern_valid" flag: Transition to "reachable" state     [ OK ]
 TEST: IPv6 "extern_valid" flag: Transition back to "stale" state    [ OK ]
 TEST: IPv6 "extern_valid" flag: Forced garbage collection           [ OK ]
 TEST: IPv6 "extern_valid" flag: Periodic garbage collection         [ OK ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250626073111.244534-3-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>