feat(@embark/testing): introduce plugin APIs to register compilers

Embark relies on certain specific plugin properties, e.g. registered compilers,
and retrieves them using `plugins.getPluginProperties('compilers', 'compilers')`.

In order to make this work in the testing environment, we need those same APIs
in the `embark-testing` package as well.

This commit adds necessary APIs to `Plugins` and `Plugin` to make registering and
loading compiler plugins work.

packages/utils/testing/src/lib/plugin.js

@@ -1,16 +1,25 @@
 class Plugins {
   constructor() {
     this.plugin = new Plugin();
+    this.plugins = [];
   }
 
-  createPlugin() {
-    return this.plugin;
+  createPlugin(name) {
+    let plugin = new Plugin({ name });
+    this.plugins.push(plugin);
+    return plugin;
   }
 
   emitAndRunActionsForEvent(name, options, callback) {
+    this.runActionsForEvent(name, options, callback);
+  }
+
+  runActionsForEvent(name, options, callback) {
     const listeners = this.plugin.getListeners(name);
     if (listeners) {
       listeners.forEach(fn => fn(options, callback));
+    } else {
+      callback(null, options);
     }
   }
 
@@ -20,12 +29,27 @@ class Plugins {
 
   teardown() {
     this.plugin.listeners = {};
+    this.plugins.forEach(plugin => plugin.teardown());
+  }
+
+  getPluginsProperty(pluginType, prop, childProp) {
+    let plugins = this.plugins.filter(plugin => plugin.has(pluginType));
+
+    let properties = plugins.map(plugin => {
+      if (childProp) {
+        return plugin[prop][childProp];
+      }
+      return plugin[prop];
+    });
+    return properties.length > 0 ? properties.reduce((a,b) => { return a.concat(b); }) || [] : [];
   }
 }
 
 class Plugin {
   constructor() {
     this.listeners = {};
+    this.pluginTypes = [];
+    this.compilers = [];
   }
 
   getListeners(name) {
@@ -38,6 +62,24 @@ class Plugin {
     }
     this.listeners[name].push(action);
   }
+
+  has(pluginType) {
+    return this.pluginTypes.indexOf(pluginType) >= 0;
+  }
+
+  addPluginType(pluginType) {
+    this.pluginTypes.push(pluginType);
+    this.pluginTypes = Array.from(new Set(this.pluginTypes));
+  }
+
+  registerCompiler(extension, cb) {
+    this.compilers.push({extension: extension, cb: cb});
+    this.addPluginType('compilers');
+  }
+
+  teardown() {
+    this.compilers = [];
+  }
 }
 
 module.exports = Plugins;