test(NetworkManagerHUD): add unit test

[Nietod]

Added unit tests for the NetworkManagersHUDs lifecycle and attributes.

Not sure about the OnGUI Methods. Found some workaround by using delegates but could be overengineered and not suitable.

// NetworkManagerHUD
internal Func<string, GUILayoutOption[], bool> button = GUILayout.Button;

internal void OnGUI()
{
  if (button("Host", null))
  {
    //Methods...
  }
}

// NetworkManagerHUDTest
hud.button = (label, options) => true;
hud.OnGUI();

hud.button = (label, options) => false;
hud.OnGUI();

Guess can't really do OnGUI within EditModeTests.

Making this one Draft until clear instructions on how to handle OnGUI.