minor: Fix sorting issue in pitest survival checking script

.ci/pitest-survival-check-xml.groovy

@@ -119,7 +119,7 @@ private static int checkPitestReport(String profile, String flag) {
 private static Set<Mutation> getSurvivingMutations(Node mainNode) {
 
     final List<Node> children = mainNode.children()
-    final Set<Mutation> survivingMutations = new TreeSet<>()
+    final Set<Mutation> survivingMutations = new HashSet<>()
 
     children.each { node ->
         final Node mutationNode = node as Node
@@ -142,7 +142,7 @@ private static Set<Mutation> getSurvivingMutations(Node mainNode) {
  */
 private static Set<Mutation> getSuppressedMutations(Node mainNode) {
     final List<Node> children = mainNode.children()
-    final Set<Mutation> suppressedMutations = new TreeSet<>()
+    final Set<Mutation> suppressedMutations = new HashSet<>()
 
     children.each { node ->
         final Node mutationNode = node as Node
@@ -311,7 +311,7 @@ private static void printMutation(String flag, Mutation mutation) {
  */
 private static Set<Mutation> setDifference(final Set<Mutation> setOne,
                                            final Set<Mutation> setTwo) {
-    final Set<Mutation> result = new HashSet<>(setOne)
+    final Set<Mutation> result = new TreeSet<>(setOne)
     result.removeIf { mutation -> setTwo.contains(mutation) }
     return result
 }