glsl/tests: reimplement warnings-test in python

glsl/tests: reimplement warnings-test in python

This reimplements the test in python with a shell script wrapper that
allows autotools to continue to run the test without realizing that
anything has changed.

Using python has two advantages, first it's portable so this test can be
run on windows as well as Linux since it just requires python, no more
diff, pwd or sh. It's also no longer tied to autotools implementation
details, like the environment variables $srcdir and $abs_builddir,
though the autotools shell wrapper still uses those, which makes it
possible to run the test in meson.

v2: - Use $PYTHON2 in script to be consistent with other scripts in mesa

Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>