[testlib] fix bug in backup file name generation: it may erroneously replace '-' by '_' in database **parent directories**. This should fix random error when test are run under apycot.

Upstream Author: 

  %(author)s <%(author-email)s>

Copyright:

Copyright (c) %(year)s %(author)s.
%(author-web-site)s -- mailto:%(author-email)s