tests: add reproducer for Mesa issue 2946

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <!325>
5 jobs for master in 3 minutes and 43 seconds
Status Job ID Name Coverage
  Container
passed #3521413
debian

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  Build
passed #3521414
build

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passed #3521415
py.test

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passed #3521416
tox

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  External
passed continuous-integration/appveyor/branch