[gardening] Make standalone/regress_52715_test more robust

The `vm-service` is starting somewhat concurrently with the `main` isolate
and the `main` can therefore not assume it's the only one allocating
memory in the process.

By delaying the measurement logic in `main`, it's more likely the
`vm-service` will have done it's work and reached steady memory state.

This makes the test more robust.

Change-Id: I5a641ad98ba269b8c5961ea6d20b06223621a4e2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/413641
Reviewed-by: Daco Harkes <dacoharkes@google.com>

tests/standalone/regress_52715_test.dart

@@ -2,6 +2,8 @@
 // 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=--deterministic
+
 // Formatting can break multitests, so don't format them.
 // dart format off
 
@@ -12,8 +14,10 @@ import 'dart:io';
 import 'package:expect/expect.dart';
 
 void main() async {
-  final startRss = ProcessInfo.currentRss; //# measure: ok
+  // Wait to give `vm-service` isolate to startup and initialize.
+  await Future.delayed(const Duration(seconds: 1));
 
+  final startRss = ProcessInfo.currentRss; //# measure: ok
   for (var i = 0; i < 1024; i++) {
     final subscription = Directory.systemTemp.watch().listen((event) {});
     await subscription.cancel();