Add event checks to tests

[ansermino]

Presently there is no event checks in the tests. These should be added.

Some useful code here: https://github.com/centrifuge/substrate-pallet-multi-account/blob/3a40d9a1107816ee28989bf4bbbe3f0ab6a1822d/src/lib.rs#L833-L842

The issue I was facing was when multiple events fire in a single call, only the last is returned by expect_event. We should extend the functionality to support n most recent events.

Events can be collected like this:

let events: Vec<Event> = system::Module::<Test>::events().iter().map(|e| e.event.clone()).collect();