Allow tests to specify files to be downloaded and included in output

[ewencp]

We can specify fixed log files for a service that should or should not be downloaded, but sometimes there are other files involved in tests that are defined dynamically and the service isn't directly aware of. For example, the Copycat tests echo to an input file, and then the sink Copycat worker write its output to another file. But the Copycat service never knows about these files directly since they are specified in the connector configuration.

It would be helpful for tests to somehow be able to specify additional files (per service) that should be downloaded when the test completes. This would, for example, help debug what is going wrong when the test fails since the two files are supposed to be identical.