[BigBirdFlaxTests] Make tests slow
Author: patrickvonplaten
Pull request: yes

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Reduce testing time for BigBird Flax (saves ~4 minutes)


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Comments:
This reduces overall fast testing time of BigBird to 30 seconds from 4 minutes. It's still quite a bit, but if I remove more tests (or make them slow, we don't test the Flax version anymore at all really).

_The documentation is not available anymore as the PR was closed or merged._