ScalaTest没有运行任何测试

我的build.sbt看起来像这样

lazy val scalatest =  "org.scalatest" % "scalatest_2.11" % "2.2.4" % "test"
lazy val root = (project in file(".")).
settings(
    name := "highlight2pdf",version := "0.1",scalaVersion := "2.11.6",libraryDependencies +=  scalatest
)

我只是将示例测试放在test / scala上

import collection.mutable.Stack
    import org.scalatest._

    class ExampleSpec extends FlatSpec with Matchers {

        "A Stack" should "pop values in last-in-first-out order" in {
            val stack = new Stack[Int]
            stack.push(1)
            stack.push(2)
            stack.pop() should be (2)
            stack.pop() should be (1)
        }

        it should "throw NoSuchElementException if an empty stack is popped" in {
            val emptyStack = new Stack[Int]
            a [NoSuchElementException] should be thrownBy {
                emptyStack.pop()
            } 
        }
    }

它总是显示：

[info]没有执行任何测试.

任何艰难的方式都没有用？