Fix comparison with singletons

Many plases in tests code use assertEqual(AA, None),
the same with True and False and negatives of such comparisons.

As comparison to singletons must use `is` operator, appropriate usage
is assertIs, assertIsNot, assertIsNone, assertIsNotNone

These assertions are provided by the `testtools` package,
so this change is not breaking backward compatibility with Python 2.6

This first patch inlcudes roughly half of the files touched by these changes
(up to and including test_n* sans `contrib` code) to ease the review.

Partial-Bug: #1259023

Change-Id: If6779c9230cf73ab93165a00040d1ca32d087c80