add Hint tests

test/content/hint.html

@@ -0,0 +1 @@
+<a id='test-link' href='javascript:window.vimvixenTest="hello"' >link</a>

test/content/hint.test.js

@@ -0,0 +1,58 @@
+import { expect } from "chai";
+import Hint from '../../src/content/hint';
+
+describe('Hint class', () => {
+  beforeEach(() => {
+    document.body.innerHTML = __html__['test/content/hint.html'];
+  });
+
+  describe('#constructor', () => {
+    it('creates a hint element with tag name', () => {
+      let link = document.getElementById('test-link');
+      let hint = new Hint(link, 'abc');
+      expect(hint.element.textContent.trim()).to.be.equal('abc');
+    });
+
+    it('throws an exception when non-element given', () => {
+      expect(() => new Hint(window, 'abc')).to.throw(TypeError);
+    });
+  });
+
+  describe('#show', () => {
+    it('shows an element', () => {
+      let link = document.getElementById('test-link');
+      let hint = new Hint(link, 'abc');
+      hint.hide();
+      hint.show();
+
+      expect(hint.element.style.display).to.not.equal('none');
+    });
+  });
+
+  describe('#hide', () => {
+    it('hides an element', () => {
+      let link = document.getElementById('test-link');
+      let hint = new Hint(link, 'abc');
+      hint.hide();
+
+      expect(hint.element.style.display).to.equal('none');
+    });
+  });
+
+  describe('#remove', () => {
+    it('removes an element', () => {
+      let link = document.getElementById('test-link');
+      let hint = new Hint(link, 'abc');
+
+      expect(hint.element.parentElement).to.not.be.null;
+      hint.remove();
+      expect(hint.element.parentElement).to.be.null;
+    });
+  });
+
+  describe('#activate', () => {
+    // TODO test activations
+  });
+});
+
+