[debugger] Improve unittest

Bug: v8:7584
Change-Id: I03ee3332960bf733a79f95a6cbe8563212cbaa1d
Reviewed-on: https://chromium-review.googlesource.com/995453Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52350}

test/debugger/debug/es6/debug-promises/reject-in-constructor-opt.js

@@ -57,3 +57,5 @@ foo();
 // bar does not get inlined into foo.
 foo();
 %RunMicrotasks();
+
+assertEquals(0, expected_events);