test: skip test-cgroup-id on ENOSYS from cg_cgroupid_open

Most container managers will block open_by_handle_at with seccomp to
mitigate a container escape attack. LXD in particular returns ENOSYS
rather than e.g. EPERM like nspawn. Skip this test if we get ENOSYS
from open_by_handle_at via cg_cgroupid_open.

src/test/test-cgroup.c

@@ -159,6 +159,8 @@ TEST(id) {
 
         if (ERRNO_IS_NEG_PRIVILEGE(fd2))
                 log_notice("Skipping open-by-cgroup-id test because lacking privs.");
+        else if (ERRNO_IS_NEG_NOT_SUPPORTED(fd2))
+                log_notice("Skipping open-by-cgroup-id test because syscall is missing or blocked.");
         else {
                 assert_se(fd2 >= 0);