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());
  }
}