ktest: Use config-bisect.pl in ktest.pl

ktest: Use config-bisect.pl in ktest.pl

Reduce code duplication and take advantage of bisection logic
improvements by calling config-bisect.pl.

The output of make oldconfig is now copied directly to the desired file,
rather than doing assign_configs+save_config, in order to preserve the
ordering so that diffing the configs at the end will provide useful
output.

Link: http://lkml.kernel.org/r/20170717001630.10518-8-swood@redhat.com
Signed-off-by: Scott Wood <swood@redhat.com>
[ Modified to use with new version of config-bisect.pl ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>