net/netmon: report disappeared interface IPs (#20621)

InterfaceIPDisappeared is documented as true when an address existed in the
old network state and is absent from the new network state. The
non-nil-state predicate was reversed, so it reported addresses that
appeared instead.

Fix the predicate and add table coverage for disappeared, appeared,
unchanged, and unknown-state cases.

Fixes #20614

Change-Id: Ieb7a8c5e13d23e868e3ffe0b99e20fb14ba2245e

Signed-off-by: cyphercodes <cyphercodes@users.noreply.github.com>
Co-authored-by: cyphercodes <cyphercodes@users.noreply.github.com>