[libc++][test] Make some string tests MSVC-friendly

* Using one-or-two letter names for globals is asking for shadowing warnings.
* MSVCSTL's container proxy allocations strike again
* MSVCSTL's `<string>` doesn't define `std::out_of_range`
* `basic_string::substr` takes two arguments of type `size_type`. Let's use that type instead of `size_t` and `ptrdiff_t` to avoid narrowing warnings.

Differential Revision: https://reviews.llvm.org/D141253