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/tests/misc.nix

@@ -77,7 +77,7 @@ import ./make-test-python.nix ({ pkgs, ...} : rec {
           machine.succeed('[ -x "$(cat /proc/sys/kernel/poweroff_cmd)" ]')
 
       with subtest("whether the blkio controller is properly enabled"):
-          machine.succeed('[ -n "$(cat /sys/fs/cgroup/blkio/blkio.sectors)" ]')
+          machine.succeed("[ -e /sys/fs/cgroup/blkio/blkio.reset_stats ]")
 
       with subtest("whether we have a reboot record in wtmp"):
           machine.shutdown