Cleanup use of QJSValue::engine()

Not only this causes warnings because it is deprecated, it lead to a
crash in our QML tests when the callback fails to return a valid
engine instance.

Change-Id: I9412f138f87c39fbee8dbb6121a5bf0a69dcfbb4
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>