chore: Organize Messenger tests (#6418)

## Explanation

The Messenger tests for actions and events have each been moved into a
`describe` block. Additionally, test descriptions have been updated to
avoid starting with "should", in accordance with our test naming
conventions.

## References

N/A

## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- [x] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes

Author: Gudahtt