Fix flaky ProgressContantsTest.testKeepOneProperty race condition

The testKeepOneProperty test was failing randomly due to timing issues
when multiple jobs with KEEPONE_PROPERTY finish simultaneously.

Changes:
- Replace fixed 500ms wait with condition-based waiting that ensures
  all jobs have actually started (job.inProgress is true)
- Add explicit processEvents() calls after jobs complete and after
  marker job appears to ensure all UI event processing is complete
- Increase timeout from 500ms to 3000ms for job startup check

This eliminates the race condition by:
1. Ensuring all jobs are running before signaling them to finish
2. Ensuring job completion events are processed before checking state
3. Ensuring KEEPONE cleanup logic completes before final assertion

Tested with 10 consecutive runs - all passed.

Fixes eclipse-platform#370

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

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

Author: vogella

tests/org.eclipse.ui.tests/Eclipse UI Tests/org/eclipse/ui/tests/progress/ProgressContantsTest.java

@@ -242,16 +242,20 @@ public void testKeepOneProperty() throws Exception {
 			job.schedule();
 		}
 		// ensure all jobs are started before ending all at the same time
-		processEventsUntil(null, 500);
+		processEventsUntil(() -> jobs.stream().allMatch(job -> job.inProgress), 3000);
 		for (DummyJob job : jobs) {
 			job.shouldFinish = true;
 		}
 		joinJobs(jobs, 10, TimeUnit.SECONDS);
+		// Process events to ensure job completion events are handled
+		processEvents();
 		{
 			DummyJob errorJob = new DummyJob("Last Job", new Status(IStatus.ERROR, TestPlugin.PLUGIN_ID, "error"));
 			errorJob.schedule();
 			processEventsUntil(() -> findProgressInfoItem(errorJob) != null, 3000);
 		}
+		// Additional event processing to ensure all KEEPONE logic has completed
+		processEvents();
 
 		assertEquals("Only one finished job should be kept in view", 1,
 				countBelongingProgressItems(DummyFamilyJob.class));