binary-search-tree: use Collections.singletonList

Fixes: #513

exercises/binary-search-tree/src/test/java/BSTTest.java

     public void sortsSingleElement() {
         BST<Integer> sut = new BST<>();
         List<Integer> expected = Collections.unmodifiableList(
-                Arrays.asList(4)
+                Collections.singletonList(4)
         );
 
         sut.insert(4);