lisa-test broken when running multiple test suites with a single command

Created by: bjackman

nosetests allows you to list multiple test files in a single command, but this is broken in LISA.

E.g. lisa-test tests/eas/heavy_load.py tests/eas/load_tracking.py results in the heavy_load test failing due to "cpu_idle" events being missing. Probably TestEnv's test_conf is being overwritten at the wrong time.