Oh awesome, thanks a lot for your effort!

public class Node {
  private Node next;
  
  public Node getNext() {
    return this.next;
  }
  
  private void setNext(Node next) {
    this.next = next;
  }

  public static Node createChain(int tailSize, int loopSize) {
    Node prevNode = new Node();
    Node start = prevNode;
    start.setNext(start);
    if (loopSize == 1) {
      return start;
    }
    for (int i = 1; i <= tailSize; ++i) {
      prevNode.setNext(new Node());
      prevNode = prevNode.getNext();
    }
    Node endLoop = prevNode;
    for (int i = 1; i < loopSize; ++i) {
      prevNode.setNext(new Node());
      prevNode = prevNode.getNext();
    }
    prevNode.setNext(endLoop);
    return start;
  }
}

Oh sorry, I did not state I was trying to solve it in Java.
Node also implements a util method to generate the full Structure Node.createChain(a, b);

function Node() {
  this.next;
  this.getNext = function() {
    return this.next
  // code
  }
  this.setNext = function(next) {
    this.next = next
  }
}

No, it is not possible to run the tests locally in an IDE without the Node class.

The description contains following:

// Use the `getNext()` method to get the following node.
node.getNext()

Is this not sufficient?