Add test case to check if S3 keys do work

As a part of IMPALA-3306, a test case was added to check if using invalid keys hinders Impala from accessing S3.

We now need to add a test case to check if using valid keys allow Impala to access S3. We need to be able to do this without exposing actual AWS access and secret keys in our code base, and in a way that the test passes all the time.

Ideas are welcome.