Fix qqmlecmascript::propertyVarOwnership

Fix qqmlecmascript::propertyVarOwnership

Don't rely on o = new Date in the test-case to really clear the last "reference"
on the stack. As it turns out, in v4, we sometimes allocate the result of "new Date"
in a different slot and the old slot is still around (and thus the object is
visible and marked).

Instead work with separate scopes, which requires a little helper function in JS.

Change-Id: I5f318feaccfc7d83dbe28be043a0ee83e0c16355
Reviewed-by: Lars Knoll <lars.knoll@digia.com>