[ENHANCEMENT] Properly configure eslint to work with jest (#988)

* chore: make aware eslint that we use jest

By setting up the jest environment, we no longer need to declare the
'test' global in the configuration.

* chore: don't need to import expect, it's a jest environment global

* chore: don't need to import expect when creating undefined test

test/unescapeHTML.test.js

@@ -1,4 +1,3 @@
-const expect = require('expect');
 const {unescapeHTML} = require('./_30s.js');
 
 test('unescapeHTML is a Function', () => {