You can test Collection element from release type as in the example below.
- Create collection parameter with release scope
- Create attribute to the parameter (e.g name and id)
- Add the attribute to action and add the action to your flow
- Set loop type as the collection
5. Create process and add the flow to the process
6. Select test process and in the manifest section generate manifest
7.Add values to the manifest as needed
8.Load the manifest and test your process.
I don't think you can test collection element of environment type but once you completed your test as a above you can switch the scope of the collection element parameter to environment and later add the values per environment.