A tester generally does the following task in ETL environment:
1. Review the requirements 2. Review the source data - data profiling. 3. Modify the source data to meet any requirements that weren't already met with the original source data. 4. Execute the ETL job(s)/transformation(s). 5. Validate the results. 6. Validate that the data is usable. If the data isn't usable, then you really haven't gained anything.