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ARROW-12241: [Python] Make CSV cancellation test more robust

* Make the interruptible workload larger
* Ensure KeyboardInterrupt doesn't stop the test suite

Closes #9920 from pitrou/ARROW-12241-csv-interrupt-test

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>