Based on Jerome's experiment from
https://lab.nexedi.com/nexedi/slapos/blob/master/software/proftpd/test/utils.py

Main changes:

 * tearDownClass is called in case of errors in setUpClass (mimic addCleanup
   instance method)
 * the setUpClass is split to smaller methods for ease of further development
 * full instance path is exposed for test introspection