Testsuite: The req_ghc_with_threaded_rts conditional does not work for the unregisterised backend.

Summary

I had a thread using -N for a test fail on the unregisterised backend when using req_ghc_with_threaded_rts ( https://gitlab.haskell.org/ghc/ghc/-/jobs/1839852 )

I suppose we don't set the configuration properly there somehow.

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