[debugger] fix blacklisted tests.

test/mjsunit/regress/regress-5071.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 exception = null;
 
 function listener(event, exec_state, event_data, data) {
-  assertEquals(2, exec_state.frameCount());
-  assertEquals("a", exec_state.frame(0).localName(0));
-  assertEquals("1", exec_state.frame(0).localValue(0).value());
-  assertEquals(1, exec_state.frame(0).localCount());
+  if (event != Debug.DebugEvent.Break) return;
+  try {
+    assertEquals(2, exec_state.frameCount());
+    assertEquals("a", exec_state.frame(0).localName(0));
+    assertEquals(1, exec_state.frame(0).localValue(0).value());
+    assertEquals(1, exec_state.frame(0).localCount());
+  } catch (e) {
+    exception = e;
+  }
 }
 
-Debug.setListener(listener);
-
 function f() {
   var a = 1;
   {
     let b = 2;
     debugger;
   }
 }
 
+Debug.setListener(listener);
 f();
+Debug.setListener(null);
+assertNull(exception);