[pinpoint] Move mann_whitney_u.py to models/

dashboard/dashboard/pinpoint/models/mann_whitney_u_test.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import unittest
 
-from dashboard.pinpoint import mann_whitney_u
+from dashboard.pinpoint.models import mann_whitney_u
 
 
 class MannWhitneyUTest(unittest.TestCase):
   def testBasic(self):
     self.assertAlmostEqual(
         mann_whitney_u.MannWhitneyU(range(10), range(20, 30)),
         0.00018267179110955002)
     self.assertAlmostEqual(
         mann_whitney_u.MannWhitneyU(range(5), range(10)),
         0.13986357686781267)
 
   def testDuplicateValues(self):
     self.assertAlmostEqual(mann_whitney_u.MannWhitneyU([0] * 5, [1] * 5),
                            0.0039767517097886512)
 
   def testSmallSamples(self):
     self.assertEqual(mann_whitney_u.MannWhitneyU([0], [1]), 1.0)
 
   def testAllValuesIdentical(self):
     with self.assertRaises(ValueError):
       mann_whitney_u.MannWhitneyU([0] * 5, [0] * 5)
 
 
 if __name__ == '__main__':
   unittest.main()