glsl/tests: Plumb $(PYTHON2) and $(PYTHON_FLAGS) into optimization-test.

glsl/tests: Plumb $(PYTHON2) and $(PYTHON_FLAGS) into optimization-test.

Some distributions (like Arch Linux) make /usr/bin/python Python 3,
rather than Python 2.  Since compare_ir uses /usr/bin/env python,
such systems will fail to run optimization-test, causing 'make check' to
always fail.

Automake's TESTS_ENVIRONMENT variable provides a mechanism to run
programs or set environment variables in the test environment.
Ideally, I think we would want to use AM_TESTS_ENVIRONMENT, since
TESTS_ENVIRONMENT is supposed to be user-overridable.  However, it isn't
supported using the default/serial test runner.

Fixes 'make check' on Arch Linux and Gentoo.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Matt Turner <mattst88@gmail.com>