Ensure viewport is correctly reset on term.reset

Part of #4603

src/browser/TestUtils.test.ts

@@ -324,6 +324,9 @@ export class MockViewport implements IViewport {
   public scrollLines(disp: number): void {
     this._onRequestScrollLines.fire({ amount: disp, suppressScrollEvent: false });
   }
+  public reset(): void {
+    throw new Error('Method not implemented.');
+  }
 }
 
 export class MockCompositionHelper implements ICompositionHelper {