IRunner should be an instance of EventEmitter (#24971)

* IRunner extends EventEmitter

https://github.com/mochajs/mocha/blob/master/lib/runner.js#L97

* Update index.d.ts

types/mocha/index.d.ts

@@ -1,13 +1,16 @@
-// Type definitions for mocha 5.0
+// Type definitions for mocha 5.1
 // Project: http://mochajs.org/
 // Definitions by: Kazi Manzur Rashid <https://github.com/kazimanzurrashid>
 //                 otiai10 <https://github.com/otiai10>
 //                 jt000 <https://github.com/jt000>
 //                 Vadim Macagon <https://github.com/enlight>
 //                 Andrew Bradley <https://github.com/cspotcode>
+//                 Dmitrii Sorin <https://github.com/1999>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.1
 
+/// <reference types="node" />
+
 interface MochaSetupOptions {
     // milliseconds to wait before considering a test slow
     slow?: number;
@@ -201,7 +204,7 @@ declare namespace Mocha {
     }
 
     /** Partial interface for Mocha's `Runner` class. */
-    interface IRunner {
+    interface IRunner extends NodeJS.EventEmitter {
         stats?: IStats;
         started: boolean;
         suite: ISuite;