Add EnginioReply::isFinished convenience function

The new member should tell if a reply was finished. The main use case
for it is autotest in which QTRY_VERIFY(r->isFinished()) can be used
for waiting instead of QSignalSpy.
Print a reply backendStatus in qDebug stream.
Add comment about bug in Qt that breaks our code.
Expose a private API to emit delayed replies.
Fix flakiness in EnginioModel autotest