PartRenderingEngineTests.ensureCleanUpAddonCleansUp still fails randomly

[ptziegler]

https://github.com/eclipse-platform/eclipse.platform.ui/runs/57884315250

CleanupAddon should ensure that partStack is not rendered anymore, as all childs have been removed
java.lang.AssertionError: CleanupAddon should ensure that partStack is not rendered anymore, as all childs have been removed
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.assertTrue(Assert.java:42)
	at org.eclipse.e4.ui.tests.workbench.PartRenderingEngineTests.ensureCleanUpAddonCleansUp(PartRenderingEngineTests.java:2472)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at org.eclipse.e4.ui.tests.rules.WorkbenchContextRule$1.evaluate(WorkbenchContextRule.java:62)
	Suppressed: java.lang.Throwable: Screenshot written to C:\Users\RUNNER~1\AppData\Local\Temp\org.eclipse.e4.ui.tests.workbench.PartRenderingEngineTests.ensureCleanUpAddonCleansUp.png
		at org.eclipse.test.Screenshots$ScreenshotOnFailure.failed(Screenshots.java:41)
AWT screenshot saved to: C:\Users\RUNNER~1\AppData\Local\Temp\org.eclipse.e4.ui.tests.workbench.PartRenderingEngineTests.ensureCleanUpAddonCleansUp.png

The test failure can be reproduced locally (on Linux at least) after executing the test ~30-70 times.

See:

• 64500cd
• 7a24db1
• a3fa065
• PartRenderingEngineTests test fails randomly #751