Get Python tests working under ASan/CI

[jsiwek]

i.e. address this: https://github.com/zeek/broker/blob/8899280694d8d5ad3aaa0a03cc99e4c3d3fd7887/ci/test.sh#L10-L15

Otherwise, can be a matter of luck whether problems like https://github.com/zeek/broker/pull/153#issuecomment-742038602 get caught early.