assert: fix diff color output

The color output was broken at some point and that was not detected
because it was not tested for properly so far. This makes sure the
colors work again and it adds a regression test as well.

PR-URL: https://github.com/nodejs/node/pull/19464


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>