Remove outdated test and fix migration to make build green again

[amritpurshotam]

Removes the test that checks if a user pays for an invoice that's not their own. Invoices were made payable by anyone here #1134.

Also fixed the migrations. The last merge migration wasn't downgrading for some reason. Since different tables were being operated on and this last migration hasn't been applied to the db, I just deleted it and made it sequential

[amritpurshotam]

Hey @avishkar58 Just got the build green again by fixing the last (but unapplied) migration and removing an outdated test.