Avoid depending on Promise.prototype.toString() in layout tests

LayoutTests/webaudio/audiocontext-suspend-resume-expected.txt

 Test suspend/resume for an (offline) AudioContext
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 PASS offlineContext = new OfflineAudioContext(1, durationInSeconds * sampleRate, sampleRate) did not throw exception.
 PASS offlineContext.state is "suspended"
 PASS p1 = offlineContext.suspend() did not throw exception.
-PASS [object Promise] is an instance of function Promise() { [native code] }
+PASS p1 is an instance of Promise
 PASS offlineContext.suspend() was correctly rejected: InvalidAccessError: cannot suspend an OfflineAudioContext
 PASS p2 = offlineContext.resume() did not throw exception.
-PASS [object Promise] is an instance of function Promise() { [native code] }
+PASS p2 is an instance of Promise
 PASS offlineContext.state is "suspended"
 PASS offlineContext.resume() was correctly rejected: InvalidAccessError: cannot resume an OfflineAudioContext
 PASS p3 = offlineContext.startRendering() did not throw exception.
 PASS offlineContext.state is "closed"
 PASS offlineContext.suspend() on a closed context rejected: InvalidAccessError: cannot suspend an OfflineAudioContext
 PASS offlineContext.resume() on a closed context rejected: InvalidAccessError: cannot resume an OfflineAudioContext
 PASS successfullyParsed is true
 
 TEST COMPLETE