Fix typos in TESTING.rst (#13169)
Fix typos in TESTING.rst (#13169)

Since TESTING.rst is not published on Apache Site, we don't run spell check on it and hence there were some typos introuduced without getting noticed.

Time to fix them
cfei18/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,airbnb/airflow,nathanielvarona/airflow,apache/incubator-airflow,apache/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,airbnb/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,apache/incubator-airflow,sekikn/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,apache/airflow,airbnb/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,mistercrunch/airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,apache/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,apache/airflow,dhuang/incubator-airflow,airbnb/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,sekikn/incubator-airflow,mistercrunch/airflow,apache/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,DinoCow/airflow,nathanielvarona/airflow,apache/airflow,DinoCow/airflow,bolkedebruin/airflow,apache/incubator-airflow,bolkedebruin/airflow