Fix tests that use a 0-length timeout.

Our infrastructure changed such that synchronous or microtask-only test
bodies run before the timeout fires, so we need to have those tests wait
for a timer event.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1654183003 .

test/runner/pause_after_load_test.dart

@@ -241,12 +241,16 @@ void main() {
 
   test("disables timeouts", () {
     d.file("test.dart", """
+import 'dart:async';
+
 import 'package:test/test.dart';
 
 void main() {
   print('loaded test 1!');
 
-  test("success", () {}, timeout: new Timeout(Duration.ZERO));
+  test("success", () async {
+    await new Future.delayed(Duration.ZERO);
+  }, timeout: new Timeout(Duration.ZERO));
 }
 """).create();
 

test/runner/runner_test.dart

@@ -410,8 +410,11 @@ import 'dart:async';
 import 'package:test/test.dart';
 
 void main() {
-  test("fail", () => throw 'oh no', onPlatform: {
-    "vm": new Timeout(new Duration(seconds: 0))
+  test("fail", () async {
+    await new Future.delayed(Duration.ZERO);
+    throw 'oh no';
+  }, onPlatform: {
+    "vm": new Timeout(Duration.ZERO)
   });
 }
 ''').create();
@@ -517,7 +520,10 @@ import 'dart:async';
 import 'package:test/test.dart';
 
 void main() {
-  test("fail", () => throw 'oh no');
+  test("fail", () async {
+    await new Future.delayed(Duration.ZERO);
+    throw 'oh no';
+  });
 }
 ''').create();