selftests/kexec: move the IMA kexec_load selftest to selftests/kexec

As requested move the existing kexec_load selftest and subsequent kexec
tests to the selftests/kexec directory.

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