Add better assert message when formatter tests fail
Add better assert message when formatter tests fail

The output of self.assertEqual is very hard to read
for code comparison. Add self.assertCodeEqual which
prints the expected and actual code in an easy to
read way.

lucius-feng/yapf,elviswf/yapf,sbc100/yapf,abduld/yapf,jamesblunt/yapf,google/yapf,wklken/yapf,google/yapf,nikolamilosevic86/yapf,dmoliveira/yapf,ambv/yapf,bygloam/yapf,thurday/yapf,hayd/yapf,sbc100/yapf