assertTrue(mock.called) -> mock.assert_called

assertTrue(mock.called) -> mock.assert_called

When checking whether a mocked method has been called,
mock.assert_called or mock.assert_called_once should be
used, and not unittest.assertTrue(mock.called)

This change does this update.
It can be verified with:
  grep -irn "self.assertTrue(.*called)" --exclude-dir=.tox
And the replacement was done with:
  sed -i 's/self.assertTrue(\(.*\).called)/\1.assert_called_once()/g' <list_of_files>

Change-Id: I4f26e0c736bf33e0b2413c8e8c33dbdb91f090e2
JIRA: YARDSTICK-865
Signed-off-by: Emma Foley <emma.l.foley@intel.com>