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MINOR: [Python] Make ARROW_DEBUG_MEMORY_POOL test more robust

When writing out of bounds and ARROW_DEBUG_MEMORY_POOL isn't set, the test subprocess may crash depending on the underlying allocator.

Closes #12595 from pitrou/minor-test-memory-py

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