test: make iterate_with() conform to declared interface

According to expirationd documentation user's function iterate_with must
accept task object and return an iterator. Patch fixes iterate_with
function defined for testing needs.

Closes #84

test/helper.lua

@@ -99,8 +99,13 @@ function helpers.truncate_spaces(g)
     g.vinyl:truncate()
 end
 
-function helpers.is_expired_true()
-    return true
+function helpers.is_expired_true(task)
+    return task.index:pairs(task.start_key(), { iterator = task.iterator_type })
+       :take_while(
+            function()
+                return task:process_while()
+            end
+        )
 end
 
 helpers.iteration_result = {}