Add regression fixture for single promoted property among plain params

[TomasVotruba]

Verifies StandaloneLinePromotedPropertyFixer splits all constructor params onto their own lines even when only one of them is promoted.

 public function __construct(
-    CorePermissions $security, Translator $translator, RouterInterface $router,
+    CorePermissions $security,
+    Translator $translator,
+    RouterInterface $router,
     private LeadModel $leadModel,
 ) {
 }

Behavior already worked; fixture locks it against regressions.