add Follow test

test/content/follow.test.js

@@ -0,0 +1,14 @@
+import { expect } from "chai";
+import Follow from '../../src/content/follow';
+
+describe('Follow class', () => {
+  describe('#codeChars', () => {
+    it('returns a string for key codes', () => {
+      let chars = [
+        KeyboardEvent.DOM_VK_0, KeyboardEvent.DOM_VK_1,
+        KeyboardEvent.DOM_VK_A, KeyboardEvent.DOM_VK_B];
+      expect(Follow.codeChars(chars)).to.equal('01ab');
+      expect(Follow.codeChars([])).to.be.equal('');
+    });
+  });
+});