nolibc test target overwites kernel config without asking

Hi,

While running the nolibc tests I discovered that they build a kernel in the current directory, including overwriting the existing .config. This is rather suprising for the selftests build system - it usually wouldn't do a kernel build at all - and might be annoying for users.

KUnit deals with this by doing it's kernel build in a .kunit directory, it'd probably be good to do something similar for nolibc.

Thanks, Mark