ktest: Add USE_OUTPUT_MIN_CONFIG to avoid prompt on make_min_config

If the file that OUTPUT_MIN_CONFIG exists then ktest.pl will prompt the
user and ask them if the OUTPUT_MIN_CONFIG should be used as the
starting point for make_min_config instead of MIN_CONFIG.

This is usually the case, and to allow the user to do so, which is
helpful if the user is creating different min configs based on tests,
and they know one is a superset of another test, they can set
USE_OUTPUT_MIN_CONFIG to one, which will prevent kest.pl from prompting
to use the OUTPUT_MIN_CONFIG and it will just use it.

If USE_OUTPUT_MIN_CONIFG is set to zero, then ktest.pl will continue to
use MIN_CONFIG instead.

The default is that USE_OUTPUT_MIN_CONFIG is undefined.

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