selftests: Distinguish between missing and non-executable

If a test was missing (e.g. wrong architecture, etc), the test runner
would incorrectly claim the test was non-executable. This adds an
existence check to report correctly.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>