Set a higher default for maxDiff
Set a higher default for maxDiff

testtools.TestCase.maxDiff is the number of characters
shown when two objects do not match for assertDictEqual,
assertMultiLineEqual, and assertSequenceEqual. The
default is 640 which is too low for comparing most dicts.

Change-Id: I1cb7b12b1a7491637b695abe76207b6db8a65fb5

openstack/oslotest,openstack/oslotest