LayeredServices.cs

public abstract class StartupBase
{
  public virtual void ConfigureService(IServiceCollection services)
  {
    // Standard registrations
    
    ConfigureEntityFramework(services);
  }
  
  public abstract void ConfigureEntityFramework(IServiceCollection services);
  
  public IConfiguration Configuration { get; set; }
}

public class Startup : StartupBase
{
  public override void ConfigureEntityFramework(IServiceCollection services)
  {
    services.AddEntityFramework()
            .AddNpgsql()
            .AddDbContext<ApplicationDbContext>(options => options.UseNpgsql(Configuration["Data:DefaultConnection:ConnectionString"]));
  }
}

public class TestStartup : StartupBase
{
  public override void ConfigureEntityFramework(IServiceCollection services)
  {
    services.AddEntityFramework()
            .AddInMemoryDatabase()
            .AddDbContext<ApplicationDbContext>(options => options.UseInMemoryDatabase());
  }
}

Test.cs

      [Fact]
      public async Task Can_Setup_Whole_Server() {
        var server = new TestServer(
            TestServer.CreateBuilder()
              ).UseStartup<TestStartup>());

        await Task.Yield();

        server.CreateClient().PostAsync("/api/children",
            new System.Net.Http.StringContent("{ Name: 'Alistair', FamilyName: 'B', Sex: 'Male', DateOfBirth: '2015-01-01' }",
              new System.Text.UTF8Encoding(), "application/json")).Wait();
        
        var response = await server.CreateClient().GetAsync("/api/children");
        var body = await response.Content.ReadAsStringAsync();
        body.Should().Contain("A");
      }