ktest: Make default build option oldconfig not randconfig

For the last time, I screwed up my ktest config file, and the build went
into the default "randconfig", blowing away the .config that I had set up.
The reason for the default randconfig was because when this was first
written, I wanted to do a bunch of randconfigs. But as time progressed,
ktest isn't about randconfig anymore, and because randconfig destroys the
config in the build directory, it's a dangerous default to have. Use
oldconfig as the default.

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