ensureCleanUpAddonCleansUp test is unstable

[iloveeclipse]

Seen in https://ci.eclipse.org/platform/job/eclipse.platform.ui/job/PR-1924/4/testReport/org.eclipse.e4.ui.tests.workbench/PartRenderingEngineTests/ensureCleanUpAddonCleansUp/

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.junit.Assert.assertFalse(Assert.java:65)
	at org.eclipse.e4.ui.tests.workbench.PartRenderingEngineTests.ensureCleanUpAddonCleansUp(PartRenderingEngineTests.java:2472)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:568)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
	at org.eclipse.e4.ui.tests.rules.WorkbenchContextRule$1.evaluate(WorkbenchContextRule.java:62)
	at org.junit.rules.TestWatcher$1.evaluate(TestWatcher.java:61)