Up to you if you want to add this, but you could simplify to:

for _, path := range []string{agent.InboundPath(), agent.ReturnPath()} {
  if err := deleteEmptyFiles(logger, agent, dl.dir, path, now, after); err != nil {
		el.Add(err)
	}
}

I like this a lot better. Pushed

We should be consistent with Cleanup above. It's confusing when different coding styles are used within a file.

Should these just return "dummy" values, like return "name" instead of calling panic?

They totally should even though they aren't being used. I'll update these.

I think these might be easier to read with fewer underscores.. Maybe something like TestCleanupEmptyFiles_notRunWhenConfigZero(t *testing.T)?

Agreed. I can clean these up here shortly.

All instances of clenaup-testing should be cleanup-testing

Totally. Good catch.

I think it'd be better to use t.TempDir() here instead because it cleans up after itself.

ioutil TempDir:

It is the caller's responsibility to remove the directory when no longer needed.

Compare to testing TempDir:

The directory is automatically removed by Cleanup when the test and all its subtests complete.

I'm not sure on this one. Since the code being tested actually cleans up the files, I'm not sure if having it cleaned up automatically is what we want. Maybe? Need to look into it.

It kind of cleans up. It cleans up the files, but leaves the directory. I think the testing package's version of the method makes more sense since it's purpose built for tests.

This one's more of a nit, but I think tests cases and their output are easier to read when testify is used. As an example, this block could be replaced by assert.Equal(t, "inbound/empty_file.ach", agent.DeletedFile). If the test fails it will print out a nicely-formatted error message that lets you quickly see why the test failed.

Good to know. I'll try it out and see how it goes. If it makes reading test output better, more informative, nicely formatted then that sounds great.