Mark coverage_test as Pass Slow and enable tracing

runtime/observatory/tests/service/coverage_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.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'dart:developer';
 
@@ skipping 84 matching lines @@
          equals([33, 1, 34, 1, 105, 2]));
 
   // Isolate
   coverage = await isolate.invokeRpcNoUpgrade('_getCoverage', {});
   expect(coverage['type'], equals('CodeCoverage'));
   expect(coverage['coverage'].length, greaterThan(100));
 },
 
 ];
 
-main(args) => runIsolateTests(args, tests, testeeConcurrent: testFunction);
+main(args) => runIsolateTests(args, tests,
+                              testeeConcurrent: testFunction,
+                              trace_service: true);