Adding subsetOf scalacheck test.

Author: msteindorfer

test/scalacheck/src/test/scala/strawman/collection/immutable/ImmutableChampHashSetProperties.scala

@@ -218,4 +218,18 @@ object ImmutableChampHashSetProperties extends Properties("immutable.ChampHashSe
     xs == ys
   }
 
+  property("smaller subsetOf larger") = forAll { (inputSet: ChampHashSet[K]) =>
+    if (inputSet.isEmpty) {
+      true
+    } else {
+      val randomSamples = scala.util.Random.nextInt(inputSet.size)
+      val randomIndices = ImmutableArray.fill(randomSamples)(scala.util.Random.nextInt(inputSet.size))
+
+      val inputArray = inputSet.toArray
+      val smallerSet = ChampHashSet.from(randomIndices.map(inputArray).toSet)
+
+      smallerSet.subsetOf(inputSet)
+    }
+  }
+
 }