ktest: Add prompt to use OUTPUT_MIN_CONFIG

ktest: Add prompt to use OUTPUT_MIN_CONFIG

If the defined OUTPUT_MIN_CONFIG in the make_min_config test exists,
then give a prompt to ask the user if they want to use that config
instead, as it is very often the case, especially when the test has been
interrupted. The OUTPUT_MIN_CONFIG is usually the config that one wants
to use to continue the test where they left off.

But if START_MIN_CONFIG is defined (thus the MIN_CONFIG is not the
default), then do not prompt, as it will be annoying if the user has
this as one of many tests, and the test pauses waiting for input, while
the user is sleeping.

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