fix(probeservices): replace ams-pg-test with backend-hel

[DecFox]

Checklist

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Description

This diff changes the ams-pg-test.ooni.org to backend-hel.ooni.org in the integration tests.

[DecFox]

It seems backend-hel.ooni.org is using a version of the backend which is incompatible with the current tests (and also the backend-fsn.ooni.org prod backend). I am going to go ahead and merge this as a quick fix and document this to fix in the future.