Add test for situation where the attribute already exists

tests/src/Drupal10/Rector/Deprecation/ActionAnnotationToAttributeRector/fixture/existing_attribute_fixture.php.inc

@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * A basic example action that does nothing.
+ *
+ * @Action(
+ *   id = "action_example_basic_action",
+ *   action_label = @Translation("Action Example: A basic example action that does nothing"),
+ *   type = "system"
+ * )
+ */
+#[\Drupal\Core\Action\Attribute\Action(id: 'action_example_basic_action', action_label: new Drupal\Core\StringTranslation\TranslatableMarkup('Action Example: A basic example action that does nothing'), type: 'system')]
+class BasicExample extends ActionBase implements ContainerFactoryPluginInterface {
+
+}
+?>
+
+-----
+<?php
+
+/**
+ * A basic example action that does nothing.
+ */
+#[\Drupal\Core\Action\Attribute\Action(id: 'action_example_basic_action', action_label: new Drupal\Core\StringTranslation\TranslatableMarkup('Action Example: A basic example action that does nothing'), type: 'system')]
+class BasicExample extends ActionBase implements ContainerFactoryPluginInterface {
+
+}
+?>