pkg/node/manager: Fix inconsistent ipset entries handling on node update, add test non-regression test

[qmonnet]

Following a bug on the v1.13 branch in the way we update the list of ipset entries in the node manager, add a test to ensure that entries are added or removed as we expect on node updates.

This is a forward port of the test added first to branch v1.13, commit 9ec163aff6f ("pkg/node/manager: Add test to validate ipset entries"), although the test has been reworked to use a mock ipset manager instead of checking the real ipset (which required privileges).

Related: #29898

[EDIT] Added an additional commit to fix the inconsistent behaviour on ipset entry handling in the node manager.

Fix a bug that may cause traffic to the node internal IP addresses to be incorrectly masqueraded when node encryption and remote node identities are both disabled, due to an inconsistency in the node manager when handling ipset entries insertions and deletions on node updates.

[qmonnet]

/test

[qmonnet]

/test

[mhofstetter]

cli ✔️

thanks!

[qmonnet]

/test

[pchaigno]

Looks good to me, apart from the couple notes on the release note.