encapsulate deferred writing in WriteBuffer class

src/TestUtils.test.ts

@@ -19,10 +19,6 @@ import { IParams, IFunctionIdentifier } from 'common/parser/Types';
 import { ISelectionService } from 'browser/services/Services';
 
 export class TestTerminal extends Terminal {
-  writeSync(data: string): void {
-    this._writeBuffer.push(data);
-    this._innerWrite();
-  }
   keyDown(ev: any): boolean { return this._keyDown(ev); }
   keyPress(ev: any): boolean { return this._keyPress(ev); }
 }