subtestbase: Redo. useful changes

This partially re-applies f758f1a083fa49505c02a3fe0d51ed84add4b625
without the multiprocess / parallel behavior changes.

Signed-off-by: Chris Evich <cevich@redhat.com>
config: Copy prepdict once

The singleton's content and therefor prepdict content
should never change, only need to copy it one time.

Signed-off-by: Chris Evich <cevich@redhat.com>