Client should take care of cleaning up test suite files

[Javier Collado]

The fetch_cmd in a runlist is expected to retrieve that runlist in whatever way is needed (copy from somewhere else, get it from the version control system, etc.).

With the current implementation, also it has to take care of cleaning up the files that might have been left from a previous execution to avoid directory name clashes. In my opinion, the fetch_cmd should just fetch data, but do nothing with regard to cleanup.

Hence, the expected behaviour would be the client to remove those files or use some other strategy to avoid directory name clashing (append a random suffix or a timestamp to the test suite name would be acceptable as well).