Mock the plugins. (#726)

Doing this prevents the plugin-host tests from dumping a stack trace
to the console and looking like a serious failure occurred.

Author: lddubeau

package-lock.json

@@ -50,6 +50,7 @@
   },
   "devDependencies": {
     "@types/mocha": "2.2.48",
+    "@types/mockery": "^1.4.29",
     "grunt": "^1.0.2",
     "grunt-cli": "^1.2.0",
     "grunt-contrib-clean": "^1.0.0",
@@ -61,6 +62,7 @@
     "grunt-tslint": "^5.0.1",
     "istanbul": "^0.4.1",
     "mocha": "^5.0.4",
+    "mockery": "^2.1.0",
     "tslint": "^5.9.1"
   },
   "files": [

package.json

@@ -1,7 +1,21 @@
 import { Application } from '..';
 import Assert = require('assert');
+import * as mockery from 'mockery';
 
 describe('PluginHost', function () {
+  before (function () {
+    mockery.enable({
+      warnOnReplace: false,
+      warnOnUnregistered: false
+    });
+    mockery.registerMock('typedoc-plugin-1', () => {});
+    mockery.registerMock('typedoc-plugin-2', () => {});
+  });
+
+  after(function ()  {
+    mockery.disable();
+  });
+
   it('parses plugins correctly', function () {
     let app = new Application({
       plugin: 'typedoc-plugin-1,typedoc-plugin-2'