TestingWebConfig.cs

[Test]
public void then_should_return_users()
{
    // Arrange
    var reader = new ConfigurationReader();
    var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
    string a = "A";
    string b = "B";
    config.AppSettings.Settings.Remove(a);
    config.AppSettings.Settings.Remove(b);
    config.AppSettings.Settings.Add(new KeyValueConfigurationElement(a, "A"));
    config.AppSettings.Settings.Add(new KeyValueConfigurationElement(b, "B"));
    config.Save(ConfigurationSaveMode.Modified, true);
    ConfigurationManager.RefreshSection("appSettings");

    // Act
    var users = reader.GetUsers();

    // Assert
    Assert.That(users, Is.EqualTo("A,B"));
}