ktest: Parse off the directory name in useconfig for failures

When we store failures, we create a directory that has the build_type
in it. For useconfig, it also contains the name path of the config
file it uses. This unfortunately gets its own directory on failure.
Parse off the directory name when creating the directory to store
the failures.

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