tools/testing/selftests: don't assume the x bit is set on scripts

The x bit can easily get lost (patch(1) loses it, for example).

Reported-by: Ramkumar Ramachandra <artagnon@gmail.com>
Cc: Dave Young <dyoung@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>