Use assert as the assertion library for testing

Background

We want to get rid of nodeunit. See trac issue https://issues.adblockplus.org/ticket/6820 Moving to mocha will be a large patch, hard to review, so we can do it piece by piece.

Step 1, use a different assertion library, and NodeJS assert fit the bill. The test still work with this assertion library.

What to change

In all the tests, replace the assertions to use assert. Browser tests should be addressed as a second patch.