quick-shader.py output differs each build

Mesa CI is having issues with piglit because it seems the set of tests run changes on each build. I'm having a hard time visualizing the diff, but the diff after this command sequence should be empty:

touch tests/quick_shader.py
ninja
gzip -dc tests/quick_shader.xml.gz > a

touch tests/quick_shader.py
ninja
gzip -dc tests/quick_shader.xml.gz > b

diff -u a b

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