tst_qcamerabackend: Add initial blacklistings for Android, iOS and macOS

By running this test locally, several tests have turned out to be
failing. The entire qcamerabackend test needs a rework, and this patch
adds the initial blacklistings to make it pass locally on these
platforms. They do not fail in CI because no camera devices are
attached. Future patches will rework the tests and implementations.

Pick-to: 6.12
Change-Id: I0e4d28b51a2b31347780fe70fd5911cb79c0bf38
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>