glsl/tests: call create_test_cases.py in optimization-test

glsl/tests: call create_test_cases.py in optimization-test

This way, when someone modifies create_test_cases.py and forgets to
commit their changes again, people will notice.

v2: make sure we parse the right directories and check for existance the
right way.

v3 (Ken): Use $PYTHON2 instead of calling python directly.

Signed-off-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>