Migrated tests to jest

Used jest-codemods to migrate, will have to pass everything by hand before we can merge.

test2/differenceBy/differenceBy.test.js

@@ -0,0 +1,10 @@
+const expect = require('expect');
+const differenceBy = require('./differenceBy.js');
+
+test('Testing differenceBy', () => {
+  //For more information on all the methods supported by tape
+  //Please go to https://github.com/substack/tape
+  expect(typeof differenceBy === 'function').toBeTruthy();
+  expect(differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor)).toEqual([1.2]);
+  expect(differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x)).toEqual([ { x: 2 } ]);
+});