Fix internal test failures

The adminusers test did not properly initialize all subsystems in
MaxScale. The polling and DCB tests weren't updated with the changes to
the DCB closing.

server/core/test/testadminusers.cc

@@ -247,6 +247,7 @@ main(int argc, char **argv)
 
     unlink(buf);
 
+    admin_users_init();
     result += test1();
     result += test2();
     result += test3();