ktest: Require one TEST_START in config file

There has been too many times that I put in one too many SKIP
TEST_STARTs and start the test with the default randconfig by accident
that I added this to have ktest ask the user for which test they want to
run if no TEST_START is specified.

Now if I accidently start the test with all TEST_STARTs skipped, ktest
asks what test do I want to run, and I now have a chance to kill it
before it does a make mrproper on my build directory.

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