Fix the broken test

Declare a return type on `when_all_range`'s customization of `connect`
so that `v1::async_scope::detached_spawn()`'s require clause can
determine whether a `when_all_range` sender is const-ref-connectable.

include/unifex/when_all_range.hpp

@@ -307,7 +307,7 @@ struct _sender<Sender>::type final {
       friend auto tag_invoke(
           unifex::tag_t<unifex::connect> /* unused */,
           Sender2&& sender,
-          Receiver&& receiver) {
+          Receiver&& receiver) -> operation<remove_cvref_t<Receiver>, Sender> {
     // return an operation that wraps all connections
     return operation<unifex::remove_cvref_t<Receiver>, Sender>{
         static_cast<Receiver&&>(receiver),