[libcxx][tests] Fix order checking in unordered_multiset tests.

Some tests assume that iteration through an unordered multiset elements
will return them in the same order as at the container creation. This
assumption is not true since the container is unordered, so that no
specific order of elements is ever guaranteed for such container. This
patch introduces checks verifying that any iteration will return
elements exactly from a set of valid values and without repetition,
but in no particular order.

Thanks to Andrey Maksimov for  the patch.

Differential Revision: https://reviews.llvm.org/D56500

llvm-svn: 361494