Follow-up to commit ae6186c7e8
* require the feature in headers that should only be seen when it's
enabled,
* expose the auto-test to being run even when the feature is disabled
(the parts of that depend on the feature have needed #if-ery) and
* fix the Darwin-specific test's misguided #if-ery.
Pick-to: 6.7 6.5
Task-number: QTBUG-108199
Change-Id: I398cf44c33ffdcb4bb04f54a9d8ccfef68741e4e
Reviewed-by: Mate Barany <mate.barany@qt.io>