selftest/lkdtm: Skip stack-entropy test if lkdtm is not available

Exit with return code 4 if lkdtm is not available like other tests
in order to properly skip the test.

Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210805101236.1140381-1-misono.tomohiro@jp.fujitsu.com