selftests/exec: Rename file binfmt_script to binfmt_script.py

Rename file for readability purpose. Update its usage and references.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>