[AIRFLOW-5732] Add unit test for the version command (#6401)
[AIRFLOW-5732] Add unit test for the version command (#6401)

There's no unit test for the `airflow version` command for now. This PR
adds a unit test for that command so as to avoid an accidental
regression.
cfei18/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,airbnb/airflow,dhuang/incubator-airflow,wileeam/airflow,spektom/incubator-airflow,apache/airflow,Fokko/incubator-airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,wileeam/airflow,Acehaidrey/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,mtagle/airflow,nathanielvarona/airflow,lyft/incubator-airflow,mtagle/airflow,wileeam/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow,DinoCow/airflow,Fokko/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,airbnb/airflow,mistercrunch/airflow,airbnb/airflow,dhuang/incubator-airflow,apache/airflow,bolkedebruin/airflow,Fokko/incubator-airflow,sekikn/incubator-airflow,lyft/incubator-airflow,DinoCow/airflow,wooga/airflow,wooga/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,airbnb/airflow,apache/incubator-airflow,spektom/incubator-airflow,Acehaidrey/incubator-airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,sekikn/incubator-airflow,dhuang/incubator-airflow,apache/airflow,apache/incubator-airflow,DinoCow/airflow,wileeam/airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,wooga/airflow,mrkm4ntr/incubator-airflow,wooga/airflow,sekikn/incubator-airflow,nathanielvarona/airflow,DinoCow/airflow,lyft/incubator-airflow,mtagle/airflow,mistercrunch/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,spektom/incubator-airflow,spektom/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,Fokko/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,mtagle/airflow,bolkedebruin/airflow,apache/airflow,bolkedebruin/airflow