testing: use the copyleft-next-0.3.1 SPDX tag

Two selftests drivers exist under the copyleft-next license.
These drivers were added prior to SPDX practice taking full swing
in the kernel. Now that we have an SPDX tag for copyleft-next-0.3.1
documented, embrace it and remove the boiler plate.

Cc: Goldwyn Rodrigues <rgoldwyn@suse.com>
Cc: Kuno Woudt <kuno@frob.nl>
Cc: Richard Fontana <fontana@sharpeleven.org>
Cc: copyleft-next@lists.fedorahosted.org
Cc: Ciaran Farrell <Ciaran.Farrell@suse.com>
Cc: Christopher De Nicolo <Christopher.DeNicolo@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Thorsten Leemhuis <linux@leemhuis.info>
Cc: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Tim Bird <tim.bird@sony.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>