[libc++][test] Suppress MSVC warnings in std::expected tests

* initializing `short`s with `short`s instead of `int`s to avoid narrowing warnings
* Explicitly discard the result of `value` calls to avoid `[[nodiscard]]` warnings

Drive-by: `testException` from `value` test is duplicated in `value_or` test; remove the duplicate.

Differential Review: https://reviews.llvm.org/D141108