[AIRFLOW-XXX] Speed up DagBagTest cases (#3974)
[AIRFLOW-XXX] Speed up DagBagTest cases (#3974)

I noticed that many of the tests of DagBags operate on a specific DAG
only, and don't need to load the example or test dags. By not loading
the dags we don't need to this shaves about 10-20s of test time.


adamhaney/airflow,lyft/incubator-airflow,DinoCow/airflow,cfei18/incubator-airflow,sid88in/incubator-airflow,jgao54/airflow,Acehaidrey/incubator-airflow,apache/airflow,apache/incubator-airflow,Fokko/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,artwr/airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,r39132/airflow,dhuang/incubator-airflow,airbnb/airflow,mistercrunch/airflow,r39132/airflow,lyft/incubator-airflow,wileeam/airflow,nathanielvarona/airflow,sid88in/incubator-airflow,airbnb/airflow,spektom/incubator-airflow,apache/airflow,artwr/airflow,malmiron/incubator-airflow,sid88in/incubator-airflow,adamhaney/airflow,cfei18/incubator-airflow,artwr/airflow,Fokko/incubator-airflow,apache/incubator-airflow,spektom/incubator-airflow,fenglu-g/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,DinoCow/airflow,artwr/airflow,airbnb/airflow,cfei18/incubator-airflow,r39132/airflow,sekikn/incubator-airflow,wileeam/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,DinoCow/airflow,Fokko/incubator-airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,malmiron/incubator-airflow,apache/airflow,apache/incubator-airflow,bolkedebruin/airflow,spektom/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,fenglu-g/incubator-airflow,fenglu-g/incubator-airflow,wileeam/airflow,jgao54/airflow,wooga/airflow,danielvdende/incubator-airflow,apache/airflow,r39132/airflow,malmiron/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,adamhaney/airflow,wooga/airflow,nathanielvarona/airflow,mistercrunch/airflow,mtagle/airflow,cfei18/incubator-airflow,spektom/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,mtagle/airflow,mtagle/airflow,sid88in/incubator-airflow,airbnb/airflow,lyft/incubator-airflow,mtagle/airflow,sekikn/incubator-airflow,fenglu-g/incubator-airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,jgao54/airflow,Fokko/incubator-airflow,malmiron/incubator-airflow,wooga/airflow,wileeam/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,adamhaney/airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow,jgao54/airflow,DinoCow/airflow,nathanielvarona/airflow,bolkedebruin/airflow,wooga/airflow,mrkm4ntr/incubator-airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow