[libc++][test] zip_view test cleanups

[libc++][test] zip_view test cleanups

* The reference type of `common_input_iterator<const int*>` can't be `int&`, because an lvalue of type `const int` _can't_ bind to an `int&`. Fix by changing the return type of `operator*` to `decltype(auto)` to make it fully generic.
* `range.zip/iterator/compare.pass.cpp` verifies that the iterators of a `zip_view` don't support `<=>` when the underlying iterators do not; this is not true after LWG-3692.
* libc++ doesn't yet implement P2165R4 "Compatibility between tuple, pair and tuple-like objects", so the tests expect `zip_view` to use `pair` in places where the working draft requires `tuple`.

Differential Revision: https://reviews.llvm.org/D141216