[libcxx] Fix order checking in unordered_multimap tests.

Some tests assume that iteration through an unordered multimap 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.

Reviewed as https://reviews.llvm.org/D54838.
Thanks to Andrey Maksimov for the patch.

llvm-svn: 349780