Goal.cs

using UnityEngine;
using UnityEngine.Events;

public class Goal : MonoBehaviour {

	public Players scoresTo;
	public IScoreEvent scoreEvent;

	public void Construct(Players scoresTo, IScoreEvent scoreEvent) {
		this.scoresTo = scoresTo;
		this.scoreEvent = scoreEvent;
	}

	public void Awake() {
		this.Construct(this.scoresTo, this.scoreEvent);
	}

	public void OnTriggerEnter2D(Collider2D other) {
		if(other.tag == Tags.BALL) scoreEvent.Invoke(scoresTo);
	}

}

GoalTest.cs

using UnityEngine;
using UnityEditor;
using UnityEngine.Events;
using UnityEngine.TestTools;
using NUnit.Framework;
using NSubstitute;
using System.Collections;

public class GoalTest {

	public class OnTriggerEnter2D {
        private Goal goal;
        private IScoreEvent scoreEvent;
        private BoxCollider2D ball;

        [SetUp]
		public void BeforeEachTest() {
			goal = new GameObject().AddComponent<Goal>();
			scoreEvent = Substitute.For<IScoreEvent>();
			ball = new GameObject().AddComponent<BoxCollider2D>();
			goal.Construct(Players.ONE, scoreEvent);
		}

		[Test]
		public void Does_Nothing_When_Collider_Tag_Is_Not_Ball() {
			goal.OnTriggerEnter2D(ball);
			scoreEvent.DidNotReceive().Invoke(Arg.Any<Players>());
		}

		[Test]
		public void Calls_ScoreEvent_When_Collider_Tag_Is_Ball() {
			ball.tag = Tags.BALL;
			goal.OnTriggerEnter2D(ball);
			scoreEvent.Received().Invoke(Arg.Any<Players>());
		}
	}

}

IScoreEvent.cs

public interface IScoreEvent
{
  void Invoke(Players arg);
}

ScoreEvent.cs

[System.Serializable]
public class ScoreEvent : UnityEvent<Players>, IScoreEvent 
{ 
}