Changes:
* add `tests/lib/vendor/**/*` to `.eslintignore`
* move `qunit-composite` to `tests/lib` so that we can modify it
* move `qunit-assert-classes` to `tests/lib` so that we can modify it
* move `qunit-assert-close` to `tests/lib` so that we can modify it
* replace `assert.push` with `assert.pushResult`
* remove usage of `QUnit.extend`
Closes gh-2157
