Speed up tests that use BackfillJob (#17648)
Speed up tests that use BackfillJob (#17648)

Calling `heartbeat` was putting in a sleep in which isn't
necessary/useful in tests, where we want it to run as quick as possible.

The sleep has been kept in "normal" mode as otherwise the status output
(`[backfill progress] | finished run %s of %s |` etc.) will be
essentially spammed, rather than only being printed every few seconds.

This makes the tests/jobs/ run in about 20s (vs 120s without the
change.)
Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,apache/airflow,apache/airflow,apache/airflow,bolkedebruin/airflow,apache/incubator-airflow,apache/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,cfei18/incubator-airflow,apache/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,apache/airflow,nathanielvarona/airflow,nathanielvarona/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,bolkedebruin/airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,lyft/incubator-airflow,apache/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow