Perform user sign in via request headers for integration testing with Asp.Net Core TestHost & the Asp.Net Core Identity framework

AspNetCoreTestHostSignIn.cs

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    using (var serviceScope = app.ApplicationServices.GetRequiredService<IServiceScopeFactory>().CreateScope())
    {
        // Seed with integration tests specific data
        ServerInitialization.InitializeIntegrationTestsDatabase(app.ApplicationServices).Wait();
    }
    app.UseStaticFiles();
    app.UseIdentity();
    // To log in a user for integration tests
    // DO NOT USE IN PRODUCTION
    app.Use(next => async context =>
    {
        await SignInIntegrationTestUser(context);
        await next.Invoke(context);
    });
    app.UseMvc();
}

public async Task SignInIntegrationTestUser(HttpContext context)
{
    var integrationTestsUserHeader = context.Request.Headers["IntegrationTestLogin"];
    if (integrationTestsUserHeader.Count > 0)
    {
        var userName = integrationTestsUserHeader[0];
        var userManager = context.RequestServices.GetRequiredService<UserManager<ApplicationUser>>();
        var user = await userManager.FindByEmailAsync(userName);
        if (user == null)
        {
            return;
        }
        var signInManager = context.RequestServices.GetRequiredService<SignInManager<ApplicationUser>>();
        var userIdentity = await signInManager.CreateUserPrincipalAsync(user);
        context.User = userIdentity;
    }
}

public static HttpClient AdminClient
{
    get
    {
        var client = _testServer.CreateClient();
        client.DefaultRequestHeaders.Add("IntegrationTestLogin", "admin@example.com");
        return client;
    }
}