test: fix WPT runner cutting off inprogress workers
Having reviewed the WPT Reports I have noticed the test count varies, see e.g. here for 19.x, 20.x, and experimental, those are the ones where #47283 has landed.
This fixes the runner so that in progress multi-variant tests don't get cut off on the account of having the same filename.
https://github.com/nodejs/node/labels/fast-track because while this bug is in place we're not actually reporting on WPT failures in workers that get terminated.
I've tested this by running wpt report a number of times and getting consistent number of total reported tests.
NB: Having worked with the runner for a bit now I think a bigger refactor is imminent from my side.