Fixup TypeScript error for test (#269)

Signed-off-by: Jonathan Kingston <[email protected]>

Author: jonathanKingston

test/mock/duplex.ts

@@ -23,7 +23,7 @@ export default class MockDuplex extends Stream.Duplex {
         this.push(null);
     }
 
-    end(...args: any[]): void {
+    end(...args: any[]): this {
         this.causeEnd(); // In order to better emulate socket streams
         return (Stream.Duplex.prototype.end as any).apply(this, args);
     }