tests: Do not wipe out testing directory on error.

Addressing testers wondering like:
Only in /tmp/tmp.QMXpY8mvdu: (null)
Only in ./rds_split_simple: report-request.xml
Only in ./rds_split_simple: report.xml
The files are different after going through result data stream!
$ ls /tmp/tmp.QMXpY8mvdu/
ls: cannot access /tmp/tmp.QMXpY8mvdu/: No such file or directory