ci: fix TestOpenat2 when no systemd is used

A few cases relied on the fact that systemd is used, and thus
/sys/fs/cgroup/user.slice is available.

Guess what, in case of "make unittest" it might not be.

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

libcontainer/cgroups/file_test.go

@@ -58,8 +58,6 @@ func TestOpenat2(t *testing.T) {
 		{"/sys/fs/cgroup", "/cgroup.controllers"},
 		{"/sys/fs/cgroup/", "cgroup.controllers"},
 		{"/sys/fs/cgroup/", "/cgroup.controllers"},
-		{"/sys/fs/cgroup/user.slice", "cgroup.controllers"},
-		{"/sys/fs/cgroup/user.slice/", "/cgroup.controllers"},
 		{"/", "/sys/fs/cgroup/cgroup.controllers"},
 		{"/", "sys/fs/cgroup/cgroup.controllers"},
 		{"/sys/fs/cgroup/cgroup.controllers", ""},