[libcxx][test] the domain of == for forward iterators is iterator values from the same range

* Default-initialized `basic_string` iterators are not portably in the domain of `==`.
* Avoid comparing iterators from non-equal string_views which MSVCSTL considers not to be in the domain of equality.
* Don't test invalid range `[in, out + N)`.

Also silence some truncation warnings by testing with a non-narrowing conversion.

Differential Revision: https://reviews.llvm.org/D118049