nixos/tests/non-default-filesystems: fix btrfs mount regex (#340702)

        machine.wait_for_unit("multi-user.target")

        with subtest("BTRFS filesystems are mounted correctly"):

          machine.succeed("grep -E '/dev/vda / btrfs rw,relatime,space_cache=v2,subvolid=[0-9]+,subvol=/root 0 0' /proc/mounts")

          machine.succeed("grep -E '/dev/vda /home btrfs rw,relatime,space_cache=v2,subvolid=[0-9]+,subvol=/home 0 0' /proc/mounts")

          print("output of \"grep -E '/dev/vda' /proc/mounts\":\n" + machine.execute("grep -E '/dev/vda' /proc/mounts")[1])

          machine.succeed("grep -E '/dev/vda / btrfs rw,.*subvolid=[0-9]+,subvol=/root 0 0' /proc/mounts")

          machine.succeed("grep -E '/dev/vda /home btrfs rw,.*subvolid=[0-9]+,subvol=/home 0 0' /proc/mounts")

      '';

    };