Fix flaky ProgressContantsTest.testKeepOneProperty race condition

[vogella]

Summary

Fixes #370 - The testKeepOneProperty test was failing randomly due to timing issues when multiple jobs with KEEPONE_PROPERTY finish simultaneously.

Problem

The test exhibited flaky behavior where it would:

• Sometimes see 20 kept jobs instead of the expected 1
• Sometimes see 0 kept jobs
• Fail with "expected:<1> but was:<20>" or similar errors

The root cause was a race condition in the test's timing logic when 20 jobs with KEEPONE_PROPERTY finish at the same time.

Solution

Made three key improvements to eliminate the race condition:

1. Condition-based waiting for job startup: Changed from a fixed 500ms wait to waiting until all jobs are actually running (job.inProgress is true) with a 3000ms timeout
2. Event processing after job completion: Added explicit processEvents() call after jobs complete to ensure job completion events are handled
3. Event processing after marker job: Added another processEvents() call after the marker error job appears to ensure all KEEPONE cleanup logic has completed

Changes

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

• Line 245: Replace processEventsUntil(null, 500) with condition-based waiting
• Line 251: Add processEvents() after joinJobs()
• Line 258: Add processEvents() after marker job appears

Testing

• Ran the specific test 10 consecutive times - all passed
• Ran all tests in ProgressContantsTest class - all passed
• No test regressions introduced

Test Plan

• CI tests pass (especially on Windows and MacOS where the issue was reported)
• No regressions in other progress view tests

🤖 Generated with Claude Code

[github-actions]

Test Results

 3 018 files  ±0   3 018 suites  ±0   2h 18m 47s ⏱️ + 1m 9s
 8 234 tests ±0   7 985 ✅ ±0  249 💤 ±0  0 ❌ ±0 
23 622 runs  ±0  22 828 ✅ ±0  794 💤 ±0  0 ❌ ±0 

Results for commit 53671b4. ± Comparison against base commit 37ad17d.

♻️ This comment has been updated with latest results.

[vogella]

LGTM