[libcxx] Add missing includes in tests

A bunch of unordered containers tests call library functions but don't directly
include the corresponding header files:

- fabs() (defined in <cmath> which is not included);
- is_permutation() (defined in <algorithm> which is not included);
- next() (defined in <iterator> which is not included).
- As a result, these tests won't compile against some conformant libraries.

Reviewed as https://reviews.llvm.org/D54643.
Thanks to Andrey Maksimov for the patch.

llvm-svn: 347085