tstest/integration: add Windows unattended-mode test coverage

Follow-up to #20464; related to #20565.

On Windows, LocalBackend resets its state when the last client disconnects, unless unattended mode is enabled (or an IPN bus subscription is kept active). #20565's Windows integration tests pass `--unattended` on `tailscale up` so single-node tests keep their state across the CLI exiting.

Two coverage gaps to address:

1. **Run most tests with unattended mode both enabled and disabled.** Most tests shouldn't depend on unattended mode being on or off, and there's value in confirming the tested functionality works correctly in both configurations.
2. **Add dedicated coverage for unattended mode itself**, verify that state is cleared when the client disconnects with unattended mode disabled, and persists when unattended mode is enabled.