Test for a subtle pre-5.10 bug. Before 5.10 the overloading flag was

Test for a subtle pre-5.10 bug. Before 5.10 the overloading flag was
stored on the reference rather than the referent. Despite the fact that
objects can only be accessed via references (even internally), the
referent actually knows that it's blessed, not the references. So
taking a new, unrelated, reference to it gives an object. However, the
overloading-or-not flag was on the reference prior to 5.10, and taking
a new reference didn't (use to) copy it (prior to 5.8.9).
So test that the bug can't return - overloading should work on a
reference to something already blessed into a package with overloading.

p4raw-id: //depot/perl@32016