Adds new QUnit assertion ("not") complementary to built-in "ok"

Strangely, almost all QUnit assertions have complementary, negated version (`equal` and `notEqual` etc.) except of the basic `ok` assertion. Having to use `ok(!something);` all the time is not very readable and quite error prone (exclamation mark is easy to miss) - I was bitten by it at least a few times when writing JS tests. This PR adds a complementary assertion `not` (that is just a negated `ok`). It makes possible asserting `not(something);` what is much more readable than `ok(!something);`