So connectivity-test.ext.nokia.com was 403 in May 2015, but as long as it was returning 204 in reply to the X-Nokia-WLAN-Connectivity-Test header, it should have been OK. (Do we know it ever returned anything other than 403 to a request without the header?)

In December 2015, connectivity-test.ext.nokia.com was not responding at all.

I've checked my server logs for the last week and I can confirm now that if a wifi connection is created and the connectivity test is successful, that wifi connection is never tested again.
If a wifi connection is created and the connectivity test fails (or connectivity-test.ext.nokia.com is down), the connectivity test is run every time that wifi connection is established, and whenever it fails again the connection is changed to Manual.

There must be some setting stored on the N9, and it must be possible to change it.