ktest: Copy the last good and bad configs in config_bisect

During the config_bisect, in case of failure, it is nice to have
the last good and bad .configs that were used. This would let
us restart the config_bisect from those configs.

Copy the last good config into the output dir as config_good,
and the last bad config as config_bad.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>