Use hydrated warning

The HydrationChecker uses the useHydrated hook but in a way that generates a warning. This MR hides that warning.

I also fixed all other warnings and error messages generated during the tests.

closes #30 (closed)

Reviewing this should be as easy as a quick code check and then checking the build logs on master: https://gitlab.developers.cam.ac.uk/digital/design-system/design-system/-/jobs/227186#L227
And the build logs on this branch: https://gitlab.developers.cam.ac.uk/digital/design-system/design-system/-/jobs/227500#L224
And noting that there are no Warnings or error logs.