[libcxx][NFC] Add tests for associative containers key_comp and value_comp

Add missing tests to improve associative containers code coverage:
 - Tests for key_comp() and value_comp() observers
 - Tests for std::map and std::multimap value_compare member class

Reviewed by: ldionne, rarutyun, #libc

Differential Revision: https://reviews.llvm.org/D113998