[libc++][test] Fix invalid test for views::view_interface

The type `MoveOnlyForwardRange` violates the precondition stated in
`view.interface.general`. Specifically, the type passed to
`view_interface` shall model the `view` concept. In turn, this requires the
type to satisfy `movable` concept (and others), but this type
`MoveOnlyForwardRange` does not satisfy the `movable` concept.

Add a move assignment operator so that `MoveOnlyForwardRange` satisfies the
`movable` concept. While we're here, ensure the neighboring types that inherit
from `view_interface` also satisfy the `view` concept to avoid similar issues.

Fixes https://bugs.llvm.org/show_bug.cgi?id=50720

Reviewed By: Quuxplusone, Mordante, #libc

Differential Revision: https://reviews.llvm.org/D112631