Add regression fixture for readonly promoted properties

[TomasVotruba]

Reported case: constructor with 3 private readonly promoted properties on a single line was suspected of not being split.

Verified StandaloneLinePromotedPropertyFixer already handles it — this adds a fixture to lock the behavior in.

 final class ReadonlyPromotedProperties
 {
-    public function __construct(private readonly EntityManager $entityManager, private readonly SchedulerPlanner $schedulerPlanner, private readonly SchedulerRepository $schedulerRepository)
+    public function __construct(
+        private readonly EntityManager $entityManager,
+        private readonly SchedulerPlanner $schedulerPlanner,
+        private readonly SchedulerRepository $schedulerRepository
+    )
     {
     }
 }

Note for anyone hitting this: the rule sits at index 17 of SpacesLevel::RULES, so withSpacesLevel() below 17 never loads it.