Fix timing-based flakey test in TestLocalTaskJob (#8405)
Fix timing-based flakey test in TestLocalTaskJob (#8405)

This test suffered from timing-based failures, if the "main" process
took even fractionally too long then the task process would have already
cleaned up it's subprocess, so the expected callback in the main/test
process would never be run.

This changes is so that the callback _will always be called_ in the test
process if it is called at all.
Acehaidrey/incubator-airflow,wooga/airflow,apache/airflow,airbnb/airflow,lyft/incubator-airflow,mistercrunch/airflow,DinoCow/airflow,airbnb/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,apache/airflow,mrkm4ntr/incubator-airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,wooga/airflow,mistercrunch/airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,mistercrunch/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,DinoCow/airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,DinoCow/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,wooga/airflow,airbnb/airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,cfei18/incubator-airflow,wooga/airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,dhuang/incubator-airflow,apache/airflow,apache/airflow,airbnb/airflow,cfei18/incubator-airflow,lyft/incubator-airflow