Fix flaky PartRenderingEngineTests.ensureCleanUpAddonCleansUp race condition

The test was failing intermittently on Windows CI with "CleanupAddon should
ensure that partStack is not rendered anymore, as all childs have been removed"
because it was checking the cleanup result before the async cleanup logic had
a chance to execute.

Root cause:
- CleanupAddon performs cleanup asynchronously via Display.asyncExec()
  (CleanupAddon.java:352)
- The test called contextRule.spinEventLoop() immediately after hiding parts
  (line 2469), which processes only currently queued events
- This created a race condition: spinEventLoop() might process events before
  CleanupAddon's asyncExec() callback was even added to the event queue
- The premature event processing caused timing issues that made the subsequent
  DisplayHelper.waitForCondition() unreliable

Fix:
- Remove the contextRule.spinEventLoop() call on line 2469
- Rely solely on DisplayHelper.waitForCondition(), which properly handles event
  processing via Display.sleep() with a 10ms retry interval
- Simplify the condition from "isToBeRendered() == false" to "!isToBeRendered()"

This follows the pattern from recent race condition fixes:
- Commit 55574d8: "removing premature event processing" for RCPTestWorkbenchAdvisor
- Commit 7c0684c: Using DisplayHelper.waitForCondition for async operations

Verified with 5 consecutive test runs - all passed consistently (0.028-0.036s).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: vogella

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

@@ -2466,11 +2466,13 @@ public void ensureCleanUpAddonCleansUp() {
 		assertTrue(" PartStack with children should be rendered", partStackForPartBPartC.isToBeRendered());
 		partService.hidePart(partB);
 		partService.hidePart(partC);
-		contextRule.spinEventLoop();
+		// DisplayHelper.waitForCondition() handles event processing via Display.sleep()
+		// and retries. Calling spinEventLoop() here creates a race condition where
+		// events may be processed before CleanupAddon's asyncExec() is queued (line 352).
 		assertTrue(
 				"CleanupAddon should ensure that partStack is not rendered anymore, as all childs have been removed",
 				DisplayHelper.waitForCondition(Display.getDefault(), 5_000,
-						() -> partStackForPartBPartC.isToBeRendered() == false));
+						() -> !partStackForPartBPartC.isToBeRendered()));
 		// PartStack with IPresentationEngine.NO_AUTO_COLLAPSE should not be removed
 		// even if children are removed
 		partService.hidePart(editor, true);