We already check this for cases where ipn.Prefs.AutoExitNode is configured via syspolicy.
Configuring it directly through EditPrefs should behave the same, so we add a test for that as well.
Additionally, we clarify the implementation and future extensibility in (*LocalBackend).resolveAutoExitNodeLocked,
where the AutoExitNode is actually enforced.
Updates tailscale/corp#29969
Updates #cleanup
Signed-off-by: Nick Khyl <nickk@tailscale.com>