Add WebApplicationFactory.CreateServer for use from built service collection

[BrennanConroy]

Background and Motivation

While moving MVC tests from WebHostBuilder to HostBuilder, I noticed a couple tests expecting CreateServer to be called but it wasn't because in the HostBuilder case we were constructing the TestServer instance after the HostBuilder was built.

So it seemed logical to add a new overload that let you create the TestServer instance after the host was built.

Proposed API

public partial class WebApplicationFactory<TEntryPoint> : IDisposable, IAsyncDisposable where TEntryPoint : class
{
     protected virtual TestServer CreateServer(IWebHostBuilder builder) => new(builder);

+    protected virtual TestServer CreateServer(IServiceProvider serviceProvider) => new(serviceProvider);
}

Usage Examples

public class TestServerFactory<TStartup> : WebApplicationFactory<TStartup>
    where TStartup : class
{
    // other methods ...

    protected override TestServer CreateServer(IServiceProvider serviceProvider)
    {
        // potentially custom configuration
        return new(serviceProvider);
    }
}

Alternative Designs

Risks

Two CreateServer overloads now exist. So old test code would need to update to use the new overload now/as well.

[dotnet-policy-service]

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.