Use 'shoulda' for more concise tests

this is good clean-up..:+1:

we need to think about how we communicate what exactly we're testing. in these 2 lines, i can understand that the first is testing "be invalid if the organisation does not exist". but the second test, by itself, doesn't explicitly communicate that: "be invalid if one of the organisations does not exist". the setup for these assertions being away also doesn't help.

while we're making this switch, can we create context blocks to group `setup` and shoulda assertions together so that it conveys intent better together?