[libcxx][test] Add missing includes and suppress warnings

... from testing with MSVC's STL. Mostly truncation warnings and variables that are only used in `LIBCPP_ASSERT`.

Differential Revision: https://reviews.llvm.org/D116878

libcxx/test/std/strings/basic.string/string.capacity/resize_and_overwrite.pass.cpp

@@ -13,6 +13,7 @@
 // template<class Operation>
 // void resize_and_overwrite(size_type n, Operation op)
 
+#include <algorithm>
 #include <cassert>
 #include <string>