selftests/kexec: define common logging functions

Define log_info, log_pass, log_fail, and log_skip functions.

Suggested-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>