tst_pagepool: Revert & Fix test properly

The previous attempt was "fixing" the test without understanding the underlying issue. A change to QList value type properties handling in QML caused this test to iterate over a mutable array-like structure which holds a reference to its originating object and it changes every time the underlying property changes.

We still need to iterate over all the items here, so we make a shallow immutable copy with the usual Array.prototype.slice call beforehand.

Bonus chapter: Port testing code from verify(==) to compare(). This will produce better diagnostics.