Add the ability to run test suites in parallel.

test/runner/browser/compact_reporter_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 @TestOn("vm")
 
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 import 'package:test/src/util/io.dart';
@@ skipping 17 matching lines @@
 import 'dart:async';
 
 import 'package:path/path.dart' as p;
 import 'package:test/test.dart';
 
 void main() {
   test("success", () {});
 }
 """);
 
-    var result = _runUnittest(["-p", "chrome", "-p", "vm", "test.dart"]);
+    var result = _runUnittest(
+        ["-p", "chrome", "-p", "vm", "-j", "1", "test.dart"]);
     expect(result.stdout, contains("[VM]"));
     expect(result.stdout, contains("[Chrome]"));
     expect(result.exitCode, equals(0));
   });
 }
 
 ProcessResult _runUnittest(List<String> args) =>
     runUnittest(args, workingDirectory: _sandbox);