scripts: add spdxcheck.py self test

scripts: add spdxcheck.py self test

Add a script that will run spdxcheck.py through a couple of self tests to
simplify validation in the future.  The tests are run for both Python 2
and Python 3 to make sure all changes to the script remain compatible
across both versions.

The script tests a regular text file (Makefile) for basic sanity checks
and then runs it on a binary file (Documentation/logo.gif) to make sure it
works in both cases.  It also tests opening files passed on the command
line as well as piped files read from standard input.  Finally a run on
the complete tree will be performed to catch any other potential issues.

Link: http://lkml.kernel.org/r/20181212131210.28024-2-thierry.reding@gmail.com
Signed-off-by: Thierry Reding <treding@nvidia.com>
Thomas Gleixner <tglx@linutronix.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Joe Perches <joe@perches.com>
Cc: Jeremy Cline <jcline@redhat.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>