HDDS-13296. Integration check always passes due to missing output

[adoroszlai]

What changes were proposed in this pull request?

1. Fail checks if output.log is missing.
   • Save output.log for rat check to prevent unintended failure.
2. Remove clean from test execution, since it removes $REPORT_DIR including output.log. It is no longer necessary after HDDS-13179.
3. Fix the error pattern, BUILD FAILURE cannot happen due to --fail-never (HDDS-12500)

https://issues.apache.org/jira/browse/HDDS-13296

How was this patch tested?

Verified (1) with only a58a717:

$ ./hadoop-ozone/dev-support/checks/integration.sh -Dtest='TestConfigFileAppender' -am -pl :hdds-config
...
[INFO] BUILD SUCCESS
...

$ echo $?
1

$ cat target/integration/summary.txt 
Unknown failure, output.log missing

CI correctly reports integration test failures:
https://github.com/adoroszlai/ozone/actions/runs/15752841577

[adoroszlai]

Merged as part of #8662.