Fix race condition in ProgressContantsTest.testKeepOneProperty

The test was failing intermittently with "expected:<1> but was:<0>" because
it was asserting the KEEPONE property result before the async cleanup logic
had completed. The previous fix added an error job as a synchronization
marker, but this wasn't sufficient as the cleanup can still be pending after
the error job appears.

This fix adds explicit waits for the actual condition being tested: that
exactly 1 item from the DummyFamilyJob family remains in the progress view.
By waiting for countBelongingProgressItems() to return 1, we ensure the
KEEPONE cleanup has fully stabilized before asserting.

Fixes: https://github.com/eclipse-platform/eclipse.platform.ui/runs/55056915330

🤖 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

@@ -223,6 +223,8 @@ public void testKeepOneProperty() throws Exception {
 			errorJob.schedule();
 			processEventsUntil(() -> findProgressInfoItem(errorJob) != null, 3000);
 		}
+		// Wait for KEEPONE cleanup to stabilize at exactly 1 item
+		processEventsUntil(() -> countBelongingProgressItems(DummyFamilyJob.class) == 1, 3000);
 
 		assertEquals("Only one finished job should be kept in view", 1,
 				countBelongingProgressItems(DummyFamilyJob.class));
@@ -254,8 +256,8 @@ public void testKeepOneProperty() throws Exception {
 			errorJob.schedule();
 			processEventsUntil(() -> findProgressInfoItem(errorJob) != null, 3000);
 		}
-		// Additional event processing to ensure all KEEPONE logic has completed
-		processEvents();
+		// Wait for KEEPONE cleanup to stabilize at exactly 1 item
+		processEventsUntil(() -> countBelongingProgressItems(DummyFamilyJob.class) == 1, 5000);
 
 		assertEquals("Only one finished job should be kept in view", 1,
 				countBelongingProgressItems(DummyFamilyJob.class));