Migrated tests to jest

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

test2/flatten/flatten.test.js

@@ -0,0 +1,10 @@
+const expect = require('expect');
+const flatten = require('./flatten.js');
+
+test('Testing flatten', () => {
+  //For more information on all the methods supported by tape
+  //Please go to https://github.com/substack/tape
+  expect(typeof flatten === 'function').toBeTruthy();
+  expect(flatten([1, [2], 3, 4])).toEqual([1, 2, 3, 4]);
+  expect(flatten([1, [2, [3, [4, 5], 6], 7], 8], 2)).toEqual([1, 2, 3, [4, 5], 6, 7, 8]);
+});