How to use ADL in Constraints?

The way this sort of thing works in Ranges is by creating a Customization Point Object. This closely mirrors your second option (we stick a using-declaration in a custom namespace) except we also provide an mechanism for users to call the correct isinf without having to write a bunch of the same kind of boilerplate themselves.

A customization point object for isinf would look something like this:

namespace N {
    // make our own namespace
    namespace impl {
        // ... where we can bring in std::isinf
        using std::isinf;

        struct isinf_t {
            // our type is constrained on unqualified isinf working
            // in a context where std::isinf can be found
            template <typename T>
                requires requires (T t) {
                    { isinf(t) } -> std::same_as<bool>;
                }
            constexpr bool operator()(T t) const {
                // ... and just invokes that (we know it's valid and bool at this point)
                return isinf(t);
            }
        };
    }

    // we provide an object such that `isinf(x)` incorporates ADL itself
    inline constexpr auto isinf = impl::isinf_t{};
}

And now that we have an object, a concept follows directly:

template <typename T> 
concept Test = requires (T t) {
    N::isinf(t);
}

This is precisely how the range concept is specified.