[debugger] Further stepping support in test wrapper

test/mjsunit/debug-step-into-json.js

-// Copyright 2015 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
-
-Debug = debug.Debug
-
-var exception = null;
-var break_count = 0;
-
-function listener(event, exec_state, event_data, data) {
-  if (event != Debug.DebugEvent.Break) return;
-  try {
-    print(event_data.sourceLineText());
-    assertTrue(
-        event_data.sourceLineText().indexOf(`Break ${break_count++}.`) > 0);
-    exec_state.prepareStep(Debug.StepAction.StepIn);
-  } catch (e) {
-    exception = e;
-  }
-};
-
-function toJsonCallback() {
-  return "x";                    // Break 2.
-}                                // Break 3.
-var o = {};
-o.toJSON = toJsonCallback;
-
-Debug.setListener(listener);
-debugger;                        // Break 0.
-var result = JSON.stringify(o);  // Break 1.
-Debug.setListener(null);         // Break 4.
-
-assertEquals('"x"', result);
-assertNull(exception);