- split tests into 3:
- test_benchmarks.py: tests all benchmarks with the most basic resultsdb backend
- test_resultsdb: tests all resultsdb backends with the most basic benchmark
- test_clusterbench: tests clusterbench_submit
- made tests more robust (deletes the results foder if it already exists)
work related to [https://bugzilla.ipr.univ-rennes.fr/show_bug.cgi?id=3958]
