Unit testing extension method adding view location expander

Introduction

I'm writing unit tests for an extension method I wrote. Its only purpose it is to wrap startup logic which extends ViewLocationExpanders list by an instance implementating IViewLocationExpander. ViewLocationExpanders is a property of RazorViewEngineOptions, which can be configured in application startup in ConfigureServices() method. I'm using XUnit 2.4.1.

Usage

Instead of:

services.Configure<RazorViewEngineOptions>(options =>
{
    options.ViewLocationExpanders.Add(new ViewLocationExpander());
});

I can use:

services.AddViewLocationExpander(new ViewLocationExpander());

ViewLocationExpander

public class ViewLocationExpander : IViewLocationExpander
{
    public IEnumerable<string> ExpandViewLocations(
        ViewLocationExpanderContext context, 
        IEnumerable<string> viewLocations)
    {
        if (context == null)
        {
            throw new ArgumentNullException(nameof(context));
        }

        if (viewLocations == null)
        {
            throw new ArgumentNullException(nameof(viewLocations));
        }

        /*
            * Note:
            * {0} = action name
            * {1} = controller name
            * {2} = area name
            */

        var newViewLocations = new string[]
        {
            // Example: '/Views/Home/_Partials/FooBar.cshtml'
            "/Views/{1}/_Partials/{0}.cshtml",
        };

        // Add new locations *AFTER* MVC default locations.
        return viewLocations.Union(newViewLocations);
    }

    public void PopulateValues(ViewLocationExpanderContext context)
    {
        context.Values["customviewlocation"] = nameof(ViewLocationExpander);
    }
}

Extension method

public static IServiceCollection AddViewLocationExpander(
    this IServiceCollection services, 
    IViewLocationExpander viewLocationExpander)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    if (viewLocationExpander == null)
    {
        throw new ArgumentNullException(nameof(viewLocationExpander));
    }

    return services.Configure<RazorViewEngineOptions>(options =>
    {
        options.ViewLocationExpanders.Add(viewLocationExpander);
    });
}

Unit tests

[Fact]
public void ExtensionMethodAddsNewViewLocationExpander()
{
    // Arrange
    var services = new ServiceCollection();
    services.AddMvc();

    // These two are required to active the RazorViewEngineOptions.
    services.AddSingleton<IHostingEnvironment, HostingEnvironment>();
    services.AddSingleton<ILoggerFactory, LoggerFactory>();

    // Act
    var serviceProvider = services.BuildServiceProvider();
    var oldOptions = serviceProvider.GetRequiredService<IOptions<RazorViewEngineOptions>>().Value;
    services.AddViewLocationExpander(new ViewLocationExpander());
    serviceProvider = services.BuildServiceProvider();
    var newOptions = serviceProvider.GetRequiredService<IOptions<RazorViewEngineOptions>>().Value;

    // Assert
    Assert.True(newOptions.ViewLocationExpanders.Count > oldOptions.ViewLocationExpanders.Count);
}

Questions

1. Am I going beyond the scope of what unit testing should include? I'm afraid that my code is actually testing the basic functions of generic collections and/or aspects of ASP.NET Core.
2. If my concern above is true, that should I write unit tests to this at all? How should it work?