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>

tools/testing/ktest/sample.conf

@@ -505,7 +505,7 @@
 #TEST = ssh user@machine /root/run_test
 
 # The build type is any make config type or special command
-#  (default randconfig)
+#  (default oldconfig)
 #   nobuild - skip the clean and build step
 #   useconfig:/path/to/config - use the given config and run
 #              oldconfig on it.