Migrated tests to jest

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

test4/isString/isString.test.js

@@ -0,0 +1,13 @@
+const expect = require('expect');
+const isString = require('./isString.js');
+
+test('Testing isString', () => {
+  //For more information on all the methods supported by tape
+  //Please go to https://github.com/substack/tape
+  expect(typeof isString === 'function').toBeTruthy();
+  expect(isString('foo')).toBe(true);
+  expect(isString('10')).toBe(true);
+  expect(isString('')).toBe(true);
+  expect(isString(10)).toBe(false);
+  expect(isString(true)).toBe(false);
+});