tests: turn check if manager cannot be intialized into macro

We need to check the same thing in multiple tests. Use a shared
macro to make it easier to update the list of errnos.

Change the errno code for "unitialized cgroup fs" for ENOMEDIUM.
Exec format error looks like something more serious.

This fixes test-execute invocation in mock.

src/basic/cgroup-util.c

@@ -2135,7 +2135,7 @@ int cg_unified(void) {
         else if (F_TYPE_EQUAL(fs.f_type, TMPFS_MAGIC))
                 unified_cache = false;
         else
-                return -ENOEXEC;
+                return -ENOMEDIUM;
 
         return unified_cache;
 }