Fix race condition in flutter test when passing --serve-observatory (#123556)

`flutter test` wasn't awaiting the `_serveObservatory` request which was
causing a race condition in the `flutter test should respect
--serve-observatory` test in `test/integration.general/test_test.dart`.

Related to https://github.com/flutter/flutter/issues/123516

packages/flutter_tools/lib/src/test/flutter_tester_device.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-
-
 import 'dart:async';
 import 'dart:io' as io; // flutter_ignore: dart_io_import;
 
@@ -180,21 +178,19 @@ class FlutterTesterTestDevice extends TestDevice {
         }
 
         logger.printTrace('Connecting to service protocol: $forwardingUri');
-        final Future<FlutterVmService> localVmService = connectToVmService(
+        final FlutterVmService vmService = await connectToVmServiceImpl(
           forwardingUri!,
           compileExpression: compileExpression,
           logger: logger,
         );
-        unawaited(localVmService.then((FlutterVmService vmservice) async {
         logger.printTrace('test $id: Successfully connected to service protocol: $forwardingUri');
         if (debuggingOptions.serveObservatory) {
           try {
-              await vmservice.callMethodWrapper('_serveObservatory');
+            await vmService.callMethodWrapper('_serveObservatory');
           } on vm_service.RPCError {
             logger.printWarning('Unable to enable Observatory');
           }
         }
-        }));
 
         if (debuggingOptions.startPaused && !machine!) {
           logger.printStatus('The Dart VM service is listening on $forwardingUri');
@@ -268,6 +264,20 @@ class FlutterTesterTestDevice extends TestDevice {
     );
   }
 
+  @visibleForTesting
+  @protected
+  Future<FlutterVmService> connectToVmServiceImpl(
+    Uri httpUri, {
+    CompileExpression? compileExpression,
+    required Logger logger,
+  }) {
+    return connectToVmService(
+      httpUri,
+      compileExpression: compileExpression,
+      logger: logger,
+    );
+  }
+
   Future<void> _startDevTools(Uri forwardingUri, DartDevelopmentService? dds) async {
     _devToolsLauncher = DevtoolsLauncher.instance;
     logger.printTrace('test $id: Serving DevTools...');

packages/flutter_tools/test/general.shard/flutter_tester_device_test.dart

@@ -14,12 +14,13 @@ import 'package:flutter_tools/src/build_info.dart';
 import 'package:flutter_tools/src/device.dart';
 import 'package:flutter_tools/src/test/flutter_tester_device.dart';
 import 'package:flutter_tools/src/test/font_config_manager.dart';
+import 'package:flutter_tools/src/vmservice.dart';
 import 'package:stream_channel/stream_channel.dart';
 import 'package:test/fake.dart';
 
-import '../src/common.dart';
 import '../src/context.dart';
 import '../src/fake_process_manager.dart';
+import '../src/fake_vm_services.dart';
 
 void main() {
   late FakePlatform platform;
@@ -251,6 +252,17 @@ class TestFlutterTesterDevice extends FlutterTesterTestDevice {
     return dds;
   }
 
+  @override
+  Future<FlutterVmService> connectToVmServiceImpl(
+    Uri httpUri, {
+    CompileExpression? compileExpression,
+    required Logger logger,
+  }) async {
+    return FakeVmServiceHost(requests: <VmServiceExpectation>[
+      const FakeVmServiceRequest(method: '_serveObservatory'),
+    ]).vmService;
+  }
+
   @override
   Future<HttpServer> bind(InternetAddress? host, int port) async => FakeHttpServer();