Fix intermittent orphan test (#18530)
Fix intermittent orphan test (#18530)

The orphan scheduler test fails intermittently likely due to
the way how pytest runs child processes. The fix is to run this
test in a fork and pass the result of the test back to the
parent process. This way we can be pretty sure that the process
has no extra children.
Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,apache/airflow,apache/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,apache/airflow,apache/incubator-airflow,apache/airflow,bolkedebruin/airflow,mistercrunch/airflow,nathanielvarona/airflow,nathanielvarona/airflow,bolkedebruin/airflow,nathanielvarona/airflow,apache/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,apache/airflow,cfei18/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,apache/airflow,mistercrunch/airflow,cfei18/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow