The SimpleTest Clone module allows existing Drupal configurations to
be cloned and tested (rather than the default SimpleTest behavior of
building a test site up from install hooks).
It is meant to serve as a placeholder module until this ability is
backported to Drupal 6.x from 7.x. There are several issues where this
is being pursued:
This module is supported by OpenSourcery.
Some very basic usage examples can be found here.