[v3] std::uninitialized_copy test

[Benjamin Kosnik]

> Definitely, thanks! The name of the new testcase seems a bit weird
> (for the FSF branches): shall we maybe refer to the original (Fedora
> or RHEL Bugzilla, I suppose) PR in a comment and then use either an
> explicative name (our current practice) or just a small number for
> the name of the testcase itself?

Yes, agreed. I couldn't think of anything descriptive for this test,
but if you can please feel free to assign it something meaningful.

-benjamin