utils/diff: fix diff_interrupt_files on Python 2

During Python 3 migration, the izip call inside diff_interrupt_files has
been replaced with a zip call (zip returning an iterator in Python 3).
Import zip from builtins to ensure that it also produces an iterator in
Python 2.

Also fix the associated unit test by importing the correct function.

wa/utils/diff.py

@@ -1,6 +1,7 @@
 import os
 import re
 
+from builtins import zip
 from future.moves.itertools import zip_longest
 
 from wa.utils.misc import as_relative, diff_tokens, write_table