nixos/tests/misc: account for cgroupv2
nixos/tests/misc: account for cgroupv2

Since #104094 (d22b3ed4bcd24dcc96fd9f99c75bb568bffe2a8c), NixOS is
using the unified cgroup hierarchy by default (aka cgroupv2).

This means the blkio controller isn't there, so we should test for
something else (e.g. the presence of the io controller).

Fixes #105581.

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