Do not confuse background job failure with a planned stop #2323

[basilevs]

Fixes #2323

The background payload has to be cancelled at the end of stress test. This was achieved with Job.cancel() method. The cancellation status from subject.delete() and createInWorkspace() was occasionally propagated to the verification stage of the test and resulted in a false positive.

[github-actions]

Test Results

 1 953 files  + 27   1 953 suites  +27   1h 25m 37s ⏱️ - 1m 22s
 4 745 tests ±  0   4 721 ✅ +  1   24 💤 ±0  0 ❌  - 1 
14 235 runs  +153  14 053 ✅ +154  182 💤 ±0  0 ❌  - 1 

Results for commit 7b9dae6. ± Comparison against base commit 48beaac.

[HeikoKlare]

With this fix, I am not able to locally reproduce the sporadic test failure anymore. After #2324 I got the following error which is obviously fixed by this change:

java.lang.AssertionError: Status CANCEL: unknown code=1 
	at org.eclipse.core.tests.resources.regression.IFileTest.testIssue2290(IFileTest.java:183)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1596)
Caused by: org.eclipse.core.runtime.CoreException: 
	... 4 more