How can I unit-test a method which is using BatchDataLoader?

[yansklyarenko]

I have a method which is using BatchDataLoader and I want to write a unit test for it. My version of HotChocolate is very old (10.5.2), but this is something I can't influence right now...

So, here is the method:

public async Task<ItemList> GetItemChildren(IResolverContext context, Key key)
{
    var dataLoader = context.BatchDataLoader<Key, ItemList>("ChildrenDataLoader", async (keys) => 
    {
       // the data is pulled from the source here
    });
    return await dataLoader.LoadAsync(key, CancellationToken.None).ConfigureAwait(false);
}

I understand that I need to somehow mock the IResolverContext. So, I do it like this (a snippet from the AutoSetup method):

var services = new ServiceCollection()
    .AddBatchQueryExecutor()
    .AddQueryExecutor()
    .AddDataLoaderRegistry()
    .BuildServiceProvider();

var dataLoaderRegistry = services.GetService<IDataLoaderRegistry>();

var context = Substitute.For<IResolverContext>();
context.Service<IEnumerable<IDataLoaderRegistry>>().Returns(new[] { dataLoaderRegistry });

return context;

Now, when I run the test, it gets to the return line, enters the LoadAsync method and then just hangs. The method under test was using CacheDataLoader before, and at that time the tests passed just fine.

I would appreciate if you have an idea what could be wrong with this setup.
Is it at all possible to mock the IResolverContext the way to make test work?