chore: fix tests on Node 18.17.0

[Conduitry]

For some bizarre reason, Node 16.17.0 is complaining that fileURLToPath is getting passed an instance of URL when it expects an instance of URL. I don't know what this could be caused by other than nodejs/node#47179, but that's still entirely mysterious. To get the tests working again, I'm instead just getting an uncontroversial string to pass to fileURLToPath to do with as it pleases.

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