Add lookup/2 to failing tests in poly_should_pass.erl

We don't want that in practice, so debugging this is another item to add to the backlog.

Author: erszcz

test/known_problems/should_pass/poly_should_pass.erl

@@ -2,7 +2,8 @@
 
 -gradualizer([solve_constraints]).
 
--export([find1/0,
+-export([lookup/2,
+         find1/0,
          l/0]).
 
 -spec lookup(T1, [{T1, T2}]) -> (none | T2).