[ package:vm_service ] Fix flaky get_memory_usage_test.dart

Fixes https://github.com/dart-lang/sdk/issues/54586

Change-Id: Ide277dc7db1fe85a35a2a7d8cc0ad7ec70c10688
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/347141
Reviewed-by: Derek Xu <derekx@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

pkg/vm_service/test/get_memory_usage_test.dart

@@ -13,7 +13,7 @@ final tests = <VMTest>[
     final result = await service.getMemoryUsage(vm.isolates!.first.id!);
     expect(result.heapUsage, isPositive);
     expect(result.heapCapacity, isPositive);
-    expect(result.externalUsage, isPositive);
+    expect(result.externalUsage, greaterThanOrEqualTo(0));
   },
   (VmService service) async {
     bool? caughtException;