Fix `jsdom`

[GCHQDeveloper81]

We're finding that if we upgrade jsdom from 27.1.0 to 27.3.0, the tests start failing with timeouts (this appears to be because the appload event no longer fires from stencil, which you can confirm by logging from the hydrated render function --> https://github.com/gchq/LD-Explorer/blob/main/test/helpers/render.ts).

Notes:

• Just because JSDOM is the library that's displaying the problem doesn't mean that's the one that needs the fix - the change required could be in another library / config file.