I got out the old JUnit book and I tried using VS Code and its Java plugin and I had a myriad of problems. I reverted back to the old DOS command line just to verify that the problem was them, not me.

Commands

javac -d bin src/boss/ScoreCollection.java

javac -cp lib\junit-4.13.jar -d bin test/boss/ScoreCollectionTest.java

javac -cp lib\junit-4.13.jar;bin -d bin test/boss/TestRunner.java

java -cp bin;lib/junit-4.13.jar;lib/hamcrest-core-1.3.jar boss.TestRunner

Result:
test(boss.ScoreCollectionTest): Not yet implemented
false

For version 2:

javac -cp bin;lib\junit-4.13.jar;lib\hamcrest-core-1.3.jar -d bin test/boss/ScoreCollectionTest.java

java -cp bin;lib/junit-4.13.jar;lib/hamcrest-core-1.3.jar boss.TestRunner

Scoreable.java

package boss;

@FunctionalInterface
public interface Scoreable {
    int getScore();
}

ScoreCollection.java

package boss;
import java.util.*;

public class ScoreCollection {
    private List<Scoreable> scores = new ArrayList<>();
    public void add(Scoreable scoreable) {
        scores.add(scoreable);
    }
    public int arithmeticMean() {
        int total = scores.stream().mapToInt(Scoreable::getScore).sum();
        return total / scores.size();
    }
}

ScoreCollectionTest.java

package boss;
import static org.junit.Assert.*;
import org.junit.*;

public class ScoreCollectionTest { 
    @Test
    public void test() {
        fail("Not yet implemented");
    }
}

ScoreCollectionTest.java Version 2

package boss;
import static org.junit.Assert.*;
import static org.hamcrest.core.IsEqual.equalTo;
import static org.hamcrest.MatcherAssert.assertThat;
import org.junit.*;

public class ScoreCollectionTest { 
    @Test
    public void answersArithmeticMeanOfTwoNumbers() {
        ScoreCollection coll = new ScoreCollection();
		coll.add(() -> 5);
		coll.add(() -> 7);
		int actualResult = coll.arithmeticMean();
		assertThat(actualResult, equalTo(6));
    }
}

TestRunner.java

package boss;
import org.junit.runner.JUnitCore;
import org.junit.runner.Result;
import org.junit.runner.notification.Failure;

public class TestRunner {
   public static void main(String[] args) {
      Result result = JUnitCore.runClasses(ScoreCollectionTest.class);
		
      for (Failure failure : result.getFailures()) {
         System.out.println(failure.toString());
      }
		
      System.out.println(result.wasSuccessful());
   }
}