Beyond isolation, this stuff is also used to test (de-)serialisation.
Much-of-a-muchness in this case since it isn't too thoroughly tested,
but best do it anyway.
The lack of this is currently failing a unit test on
`osu-server-spectator` current master:
https://github.com/ppy/osu-server-spectator/actions/runs/14193158383/job/39762243965#step:4:28
I don't think the failure actually matters because I don't think we're
using json serialisation on spectator server side anywhere (used to for
iOS at least, but I don't think we do anymore?), but probably better to
be safe than sorry.