The previous fix in commit 9c171a0411 added UITestUtil.processEvents()
to wait for decorator enablement, but this only processes the event
queue and doesn't wait for asynchronous jobs to complete.

The DecoratorManager.setEnabled() method triggers updateForEnablementChange()
which uses fireListenersInUIThread() to schedule WorkbenchJob instances.
These jobs may not complete before tests check decoration results, causing
intermittent failures.

This fix introduces waitForDecoratorJobs() which uses Job.getJobManager().join()
to wait for all FAMILY_DECORATE jobs to complete before proceeding with tests.

Fixes: https://github.com/eclipse-platform/eclipse.platform.ui/issues/868

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