Migrate client tests to Jest

[GentlemanHal]

We migrated the tests to Jest once before but then we migrated them to Mocha instead.

The reason we migrated away was because of Jests auto-mocking which made the tests quite slow and caused some issues.

In the almost 3 years since then, Jest has got faster and auto mocking is disabled by default. Also almost all developers I've talked to who've worked on React projects in the last few years have used Jest, and they find proxyquire mocking quite confusing.

Given this I think we should try using Jest again.