Create unit tests for ingester's process submission file

[MarceloRobert]

process_submissions.py defines the functions that receive submission data (dict) and creates the instances of each model in that submission.
Detached from #1569 so that we can be more granular with the tests

[joao-faria-dev]

Hey guys, I'd like to contribute on this one. Is that okay @MarceloRobert ?

[MarceloRobert]

@joao-faria-dev of course! It would be very helpful, you can take the other unit test issues as examples, #1592, #1593, #1598

Just let me know if I can assign you to the issue and move this to In Progress if you take it

cc @gustavobtflores

[joao-faria-dev]

Great, please assign it to me then