selftests: livepatch: Avoid running the tests if kernel-devel is missing

By checking if KDIR is a valid directory we can safely skip the tests if
kernel-devel isn't installed (default value of KDIR), or if KDIR
variable passed doesn't exists.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402191417.XULH88Ct-lkp@intel.com/


Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>