t_a to 'a', where 'a' is the skolem from test5's signatures (due to the
Modular s a predicate in that signature). If we don't zonk (Modular s t_a) in
the givens, we will get into a loop as improveOne uses the unification engine
-TcGadt.tcUnifyTys, which doesn't know about mutable type variables.
+Unify.tcUnifyTys, which doesn't know about mutable type variables.
Note [LOOP]