[libc++][test] Silence allocator conversion warnings

[libc++][test] Silence allocator conversion warnings

... by accepting `std::size_t` instead of `int` in `allocate` and `deallocate` functions.

Drive-by: To conform to the allocator requirements, the `Allocator` types in these tests need to have (1) converting constructors and (2) cross-specialization `==` that returns `true` at least for copies of the same allocator.
Differential Revision: https://reviews.llvm.org/D141334