I have a test class as

import org.scalatest.FlatSpec

import scala.collection.mutable

class Tags101Spec extends FlatSpec {
  "A Stack" should "pop values in last-in-first-out order" in {
    val stack = new mutable.Stack[Int]
    stack.push(1)
    stack.push(2)
    assert(stack.pop() === 2)
    assert(stack.pop() === 1)
  }

  it should "throw NoSuchElementException if an empty stack is popped" in