Add beacon API tests

[MegaRedHand]

#612

[f3r10]

could it take this one?

[MegaRedHand]

Yes, sure! (I meant to to assign it to you, not to me 😅)

[f3r10]

@MegaRedHand This is what we would need, right? https://hexdocs.pm/phoenix/testing.html

[MegaRedHand]

Yes. Although we'll probably have to mock the DB calls with patch