Avoid designating a node not running linktest as the lintest master.
Only happens in an obscure case. Since virtual nodes can be fixed to other topology nodes in the NS file, "virthost" nodes can explicitly appear in the resulting topology descriptions. However, virthost nodes do not run linktest so we need to ensure they do not get nominated as "linktest master" for an experiment. Rather than fix this on the client-side and create further backward-compat issues, we resolve this in the server-generated linktest topology map. By marking such virthost nodes as not supporting linktest, the client side will automatically weed out those nodes.