Polish CreateMockitoSpiesCallback

(cherry picked from commit 10147ea)

Author: geoand

test-framework/junit5-mockito/src/main/java/io/quarkus/test/junit/mockito/internal/CreateMockitoSpiesCallback.java

@@ -21,7 +21,7 @@ public class CreateMockitoSpiesCallback implements QuarkusTestAfterConstructCall
 
     // Set is here because in nested tests, there are multiple states created before destruction is triggered
     // This field needs to be static because each implemented callback created a new instance of this class
-    private static Set<InjectableContext.ContextState> statesToDestroy = new HashSet<>();
+    private static final Set<InjectableContext.ContextState> statesToDestroy = new HashSet<>();
 
     @Override
     public void afterConstruct(Object testInstance) {