nixosTests.misc: fix blkio sub-test with newer kernels
nixosTests.misc: fix blkio sub-test with newer kernels

The blkio.sectors file is no longer provided; by quick search:
https://issues.apache.org/jira/browse/MESOS-9848
so hopefully it's sufficient to test existence of this other file:
https://www.kernel.org/doc/html/v5.4/admin-guide/cgroup-v1/blkio-controller.html#common-files-among-various-policies

Originally this sub-test was added in 58e40f84.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs