Add and test events in `pallet-conviction-voting` (#6544)

# Description

https://github.com/paritytech/polkadot-sdk/pull/4613 introduced events
for `pallet_conviction_voting::{vote, remove_vote, remove_other_vote}`.
However:
1. it did not include `unlock`
2. the pallet's unit tests were missing an update

## Integration

N/A

## Review Notes

This is as https://github.com/paritytech/polkadot-sdk/pull/6261 was, so
it is a trivial change.