`testCached` doesn't work for `ScalaJSModule`.

[lefou]

java.lang.ExceptionInInitializerError
java.lang.ExceptionInInitializerError
        at org.scalatest.tools.TaskRunner.executionFuture(TaskRunner.scala:80)
        at org.scalatest.tools.TaskRunner.execute(TaskRunner.scala:74)
        at mill.scalalib.TestRunner$.$anonfun$runTests$2(TestRunner.scala:102)
        at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:238)
        at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:62)
        at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:55)
        at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:49)
        at scala.collection.TraversableLike.map(TraversableLike.scala:238)
        at scala.collection.TraversableLike.map$(TraversableLike.scala:231)
        at scala.collection.AbstractTraversable.map(Traversable.scala:108)
        at mill.scalalib.TestRunner$.$anonfun$runTests$1(TestRunner.scala:81)
        at mill.modules.Jvm$.inprocess(Jvm.scala:184)
        at mill.scalalib.TestRunner$.runTests(TestRunner.scala:76)
        at mill.scalalib.TestRunner$.main(TestRunner.scala:52)
        at mill.scalalib.TestRunner.main(TestRunner.scala)
Caused by: java.lang.ClassCastException: scala.scalajs.js.Object cannot be cast to scala.scalajs.js.Dictionary
        at scala.scalajs.js.Dictionary$.empty(Dictionary.scala:83)
        at scala.scalajs.reflect.Reflect$.<init>(Reflect.scala:72)
        at scala.scalajs.reflect.Reflect$.<clinit>(Reflect.scala)
        ... 15 more
1 targets failed