Try to stabilize flacky PartRenderingEngineTest

Due to an oversight, the REMOVE_ON_HIDE tag is set on "partB" twice, as
opposed to "partB" and "partC". This may occasionally keep the part
stack rendered, because "partC" is not removed from the model when
hidden.

Closes #751

Author: ptziegler

tests/org.eclipse.e4.ui.tests/src/org/eclipse/e4/ui/tests/workbench/PartRenderingEngineTests.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2009, 2023 IBM Corporation and others.
+ * Copyright (c) 2009, 2025 IBM Corporation and others.
  *
  * This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License 2.0
@@ -2445,7 +2445,7 @@ public void ensureCleanUpAddonCleansUp() {
 		partStackForPartBPartC.setSelectedElement(partB);
 
 		MPart partC = ems.createModelElement(MPart.class);
-		partB.getTags().add(EPartService.REMOVE_ON_HIDE_TAG);
+		partC.getTags().add(EPartService.REMOVE_ON_HIDE_TAG);
 		partStackForPartBPartC.getChildren().add(partC);
 		partStackForPartBPartC.setSelectedElement(partC);