tf.test.TestCase doesn't have assertLen but parametrized.TestCase does, so use that to fix broken Travis.

PiperOrigin-RevId: 219222144

Author: afrozenator

tensor2tensor/data_generators/problem_test.py

@@ -19,6 +19,7 @@
 from __future__ import division
 from __future__ import print_function
 
+from absl.testing import parameterized  # for assertLen
 import numpy as np
 
 from tensor2tensor.data_generators import algorithmic
@@ -45,7 +46,7 @@ def assert_tensors_equal(sess, t1, t2, n):
   return True
 
 
-class ProblemTest(tf.test.TestCase):
+class ProblemTest(parameterized.TestCase, tf.test.TestCase):
 
   @classmethod
   def setUpClass(cls):