TST: change assertion in reordering test

Sifting can result in different variable orders,
depending on the order of iteration over
dictionaries and sets in Python, when testing the class
`dd.autoref.BDD`, hence the class `dd.bdd.BDD`.

Instead of asserting that the optimal variable
order is obtained, assert that the size of the BDD
is reduced.

tests/common.py

@@ -680,12 +680,19 @@ def test_reorder_without_args(self):
         self._confirm_var_order(vrs, bdd)
         expr = '(z1 /\ y1) \/ (z2 /\ y2) \/ (z3 /\ y3)'
         u = bdd.add_expr(expr)
+        n_before = u.dag_size
         bdd.reorder()
-        levels = {var: bdd.level_of_var(var) for var in vrs}
-        for i in range(1, 4):
-            a = levels['z{i}'.format(i=i)]
-            b = levels['y{i}'.format(i=i)]
-            assert abs(a - b) == 1, levels
+        n_after = u.dag_size
+        assert n_after < n_before, (n_after, n_before)
+        # optimal:  n_after == 6
+        #
+        # assert that each pair zi, yi is of
+        # variables at adjacent levels
+        # levels = {var: bdd.level_of_var(var) for var in vrs}
+        # for i in range(1, 4):
+        #     a = levels['z{i}'.format(i=i)]
+        #     b = levels['y{i}'.format(i=i)]
+        #     assert abs(a - b) == 1, levels
 
     def _confirm_var_order(self, vrs, bdd):
         for i, var in enumerate(vrs):