Assert UpdatedAtNanos in neighbor entry tests
Assert UpdatedAtNanos in neighbor entry tests

Changes the neighbor_entry_test.go tests to always assert UpdatedAtNanos.

This field was historically not checked due to the lack of a deterministic,
controllable clock. This is no longer true with the tcpip.Clock interface.
While the tests have been adjusted to use Clock, asserting by the
UpdatedAtNanos was neglected.

Subsequent work is needed to assert UpdatedAtNanos in the neighbor cache tests.

Updates #4663

PiperOrigin-RevId: 359868254

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor