Improve diff for assertCountEqual() to actually show the differing counts.

New output looks like this:

Traceback (most recent call last):
  File "test.py", line 5, in test_ce
    self.assertCountEqual('abracadabra xx', 'simsalabim xx')
AssertionError: Element counts were not equal:
Expected 5, got 2:  'a'
Expected 2, got 1:  'b'
Expected 0, got 2:  'i'
Expected 0, got 2:  'm'
Expected 0, got 1:  'l'
Expected 0, got 2:  's'
Expected 1, got 0:  'c'
Expected 1, got 0:  'd'
Expected 2, got 0:  'r'

0 files changed, 0 insertions, 0 deletions