Fix test-generation when a type variable's bound is itself a type variable

For example, class G<A, B extends A>

java/ql/src/utils/GenerateFlowTestCase.qll

@@ -93,7 +93,7 @@ RefType getRootType(RefType t) {
  */
 RefType replaceTypeVariable(RefType t) {
   if t instanceof TypeVariable
-  then result = t.(TypeVariable).getFirstUpperBoundType()
+  then result = replaceTypeVariable(t.(TypeVariable).getFirstUpperBoundType())
   else result = t
 }