[libcxx][test] Fix order checking in some more 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 is a continuation of D54838 and 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/D56498

llvm-svn: 361414