selftests/filesystems: Add six consecutive 'x' characters to mktemp

In busybox, the mktemp requires that the generated filename be
suffixed with at least six consecutive 'X' characters. Otherwise,
it will return an "Invalid argument" error.

Signed-off-by: Hui Min Mina Chou <minachou@andestech.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>