fix test and improve name

src/browser/TestUtils.test.ts

@@ -254,6 +254,9 @@ export class MockBuffer implements IBuffer {
   public getWhitespaceCell(attr?: IAttributeData): ICellData {
     throw new Error('Method not implemented.');
   }
+  public clearMarkers(): void {
+    throw new Error('Method not implemented.');
+  }
 }
 
 export class MockRenderer implements IRenderer {