ktest: Fix breakage from change of oldnoconfig to olddefconfig

Commit fb16d891 "kconfig: replace 'oldnoconfig' with 'olddefconfig', and
keep the old name", changed ktest's default config update from
oldnoconfig to olddefconfig without adding oldnoconfig as a backup.
The make oldnoconfig works much better than its backup of:
   yes '' | make oldconfig

But due to this change, and the fact that ktest is used to build lots of
older kernels (and for bisects), it forgoes the oldnoconfig completely.

Cc: Adam Lee <adam8157@gmail.com>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>