glsl/tests/optimization-test: correctly manage srcdir/builddir

glsl/tests/optimization-test: correctly manage srcdir/builddir

At the moment we look for generator script(s) in builddir while they
are in srcdir, and we proceed to generate the tests and expected output
in srcdir, which is not allowed.

To untangle:
 - look for the generator script in the correct place
 - generate the files in builddir, by extending create_test_cases.py to
use --outdir

With this in place the test passes `make check' for OOT builds - would
that be as standalone or part of `make distcheck'

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>