Fix flaky PartRenderingEngineTests.ensureCleanUpAddonCleansUp race condition

The test was failing intermittently 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)
- When hidePart() is called, it triggers events that schedule async cleanup
  tasks on the event queue
- The test was calling waitForCondition() immediately after hiding parts,
  before the async cleanup tasks were even posted to the event queue
- This created a race where the wait could timeout before cleanup started

Fix:
- Add contextRule.spinEventLoop() after hiding partB and partC to ensure
  async cleanup tasks are processed before waiting for the condition
- Wrap waitForCondition() in assertTrue() to fail immediately with a clear
  message if cleanup doesn't complete within timeout
- Remove redundant assertFalse() statements

This matches the pattern used in the adjacent testBug332463 test which calls
spinEventLoop() after each hidePart() call.

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

Fixes #751

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vogella

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

@@ -2466,12 +2466,11 @@ public void ensureCleanUpAddonCleansUp() {
 		assertTrue(" PartStack with children should be rendered", partStackForPartBPartC.isToBeRendered());
 		partService.hidePart(partB);
 		partService.hidePart(partC);
-		DisplayHelper.waitForCondition(Display.getDefault(), 5_000,
-				() -> partStackForPartBPartC.isToBeRendered() == false);
-		assertFalse(
+		contextRule.spinEventLoop();
+		assertTrue(
 				"CleanupAddon should ensure that partStack is not rendered anymore, as all childs have been removed",
-				partStackForPartBPartC.isToBeRendered());
-		assertFalse("Part stack should be removed", partStackForPartBPartC.isToBeRendered());
+				DisplayHelper.waitForCondition(Display.getDefault(), 5_000,
+						() -> partStackForPartBPartC.isToBeRendered() == false));
 		// PartStack with IPresentationEngine.NO_AUTO_COLLAPSE should not be removed
 		// even if children are removed
 		partService.hidePart(editor, true);