cannot reference another script in editmode unit test

I have two files in a Unity3d project. One is a test script that runs in edit mode. The other is a single class with static functions that I'd like to call from the test scripts.

here's my test script:

 using UnityEngine;
 using UnityEngine.TestTools;
 using NUnit.Framework;
 using System.Collections;
 
 public class NewTestScript 
 {
 
     [Test]
     public void TestAnotherStaticFunction() 
     {
         int a = NewBehaviourScript.FunctionUnderTest(1);
         int b = 1;
 
         // Use the Assert class to test conditions.
         Assert.IsTrue(a == b);
     }
 }

here's my function under test:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class NewBehaviourScript : MonoBehaviour 
 {
     /// <summary>
     /// the stupidest function in the world, 
     /// used to verify tests.
     /// </summary>
     public static int FunctionUnderTest(int a)
     {
         return a;
     }
 }

This gives me the error from the Unity compiler (I'm not building outside of Unity):

Assets/TestS/NewTestScript.cs(12,17): error CS0103: The name `NewBehaviourScript' does not exist in the current context

These are running in edit-mode.

My previous unit test experience is largely in Visual Studio or VSCode, and I'm trying to get my unity3d test experience to match my prior test environment experiences.

Is there a fundamentally limited functionality in the edit-mode tests, or am I missing something stupid?

Further elaboration on the assemblies involved. It looks like there are two assemblies at play here: Assembly-CSharp.dll contains my code under test and TestS.dll contains my testing code. I believe my questions boils down to: how do I add a reference from the TestS.dll assembly to the Assembly-CSharp.dll. I'd know how to do this in Visual Studio (either via the context menu in VS or directly editing the csproj file), however I don't see how to do it in Unity3d. Any edit I make to the csproj file is frequently overwritten by unity, and while there is a 'references' section in the inspector (see picture) I can't add Assembly-CSharp.dll as a reference.