fix test that was using old code

Author: erik-krogh

javascript/ql/test/library-tests/TypeTracking/CustomStep.qll

@@ -11,9 +11,6 @@ predicate configStep(Node pred, Node succ) {
   )
 }
 
-class CustomStep extends AdditionalTypeTrackingStep, Node {
-  override predicate step(Node pred, Node succ) {
-    pred = this and
-    configStep(pred, succ)
-  }
+class CustomStep extends SharedTypeTrackingStep {
+  override predicate step(Node pred, Node succ) { configStep(pred, succ) }
 }