arb_uniform_buffer_object: produce a stable set of random tests
Seed the python's random number generator to ensure that randomized tests are reproducible from a given commit.
I checked that the shader tests have stable content with:
$ find piglit/generated_tests/spec/arb_uniform_buffer_object/execution/ -type f -name "*.shader_test" -exec md5sum {} + | awk '{print $1}' | sort | md5sum
ea139e39f04cbed7a7efd4faa5e0522b -