You need a specificy the full filepath not just the filename
exec("Add-ons/Script_Test/testing.cs");
Replace Script_Test with whatever your add-on name is
Or if you put the exec call in a file, you can exec another file in the same directory using exec("./testing.cs");
You may use this if, for example, you wish to have server.cs exec Addon_name.cs and have all the code in addon_name.cs
(Why the hell do people do that anyways? I've never done it as it seems entirely pointless to me.)