Comment 3 for bug 516934

I've got a fix.

The bug was that the feature itself and its replacement were swapped in _ensure() which led to an attempt
to probe the feature indefinitely, never thunking to the replacement.

The missing test is a bit more complicated than strictly needed due to the way the deprecation is reported
 (mentioning the replacement feature). We may want to generalize the ability to do that but that's outside the
scope of this bug.