[debugger] Further stepping support in test wrapper

test/debugger/debug/debug-generator-break-on-stack.js

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --expose-debug-as debug
 
 var Debug = debug.Debug;
 
 var break_count = 0;
 var exception = null;
 
 function listener(event, exec_state, event_data, data) {
   if (event != Debug.DebugEvent.Break) return;
   try {
     break_count++;
@@ skipping 21 matching lines @@
   Debug.setBreakPoint(g, 2);
   Debug.setBreakPoint(f, 2);
 }
 
 for (let _ of f()) { }
 
 assertEquals(2, break_count);
 assertNull(exception);
 
 Debug.setListener(null);