There is no best practice for unit testing your specific use-case. The best shortcut is probably to copy CommunityCommons.executeMicroflowAsUser_1( ) and modify its Java code to make it return the objects.
Alternatively, you could also add an input parameter to the microflow, an object of entity "System.User". In your regular calls, you can supply the microflow with the $currentUser as input parameter, whereas in your test microflow you first retrieve a user of choice and pass that to the retrieve microflow.
However, if it's going to be a datasource microflow you will also need a wrapper microflow on your page(s), which is not optimal but that's the trade-off..
Alternatively, you could also add an input parameter to the microflow, an object of entity "System.User". In your regular calls, you can supply the microflow with the $currentUser as input parameter, whereas in your test microflow you first retrieve a user of choice and pass that to the retrieve microflow.
However, if it's going to be a datasource microflow you will also need a wrapper microflow on your page(s), which is not optimal but that's the trade-off..