Just the constructor

ApplicationUsersControllerGetApplicationUser.cs

public class ApplicationUsersControllerGetApplicationUser
{
    private readonly ApplicationDbContext _context;
    private readonly HttpClient _client;

    public ApplicationUsersControllerGetApplicationUser()
    {
        var builder = new WebHostBuilder()
            .UseEnvironment("Testing")
            .UseStartup<Startup>();

        var server = new TestServer(builder);
        _context = server.Host.Services.GetService(typeof(ApplicationDbContext)) as ApplicationDbContext;
        _client = server.CreateClient();
    }
}