[libc++] Mark a failing test.

The tests fails in debug mode since it manipulates an iterator to a
`std::string` returned from the dylib. This is a known issue for the
debug iterators.

Updating the libc++ Docker image to Ubuntu Focal caused some breakage.
This was temporary disabled in D112737. This re-enables one of these
tests.

Reviewed By: ldionne, #libc, Quuxplusone

Differential Revision: https://reviews.llvm.org/D113139