net/netcheck: do not add derps if IPv4/IPv6 is set to "none"

It was documented as such but seems to have been dropped in a
refactor, restore the behavior. This brings down the time it
takes to run a single integration test by 2s which adds up
quite a bit.

Updates tailscale/corp#19786

Signed-off-by: Maisem Ali <maisem@tailscale.com>

net/netcheck/netcheck.go

@@ -450,10 +450,10 @@ func makeProbePlan(dm *tailcfg.DERPMap, ifState *netmon.State, last *Report) (pl
 			if try > 1 {
 				delay += time.Duration(try) * 50 * time.Millisecond
 			}
-			if do4 || n.IsTestNode() {
+			if n.IPv4 != "none" && (do4 || n.IsTestNode()) {
 				p4 = append(p4, probe{delay: delay, node: n.Name, proto: probeIPv4})
 			}
-			if do6 || n.IsTestNode() {
+			if n.IPv6 != "none" && (do6 || n.IsTestNode()) {
 				p6 = append(p6, probe{delay: delay, node: n.Name, proto: probeIPv6})
 			}
 		}
@@ -476,10 +476,10 @@ func makeProbePlanInitial(dm *tailcfg.DERPMap, ifState *netmon.State) (plan prob
 		for try := 0; try < 3; try++ {
 			n := reg.Nodes[try%len(reg.Nodes)]
 			delay := time.Duration(try) * defaultInitialRetransmitTime
-			if ifState.HaveV4 && nodeMight4(n) || n.IsTestNode() {
+			if n.IPv4 != "none" && ((ifState.HaveV4 && nodeMight4(n)) || n.IsTestNode()) {
 				p4 = append(p4, probe{delay: delay, node: n.Name, proto: probeIPv4})
 			}
-			if ifState.HaveV6 && nodeMight6(n) || n.IsTestNode() {
+			if n.IPv6 != "none" && ((ifState.HaveV6 && nodeMight6(n)) || n.IsTestNode()) {
 				p6 = append(p6, probe{delay: delay, node: n.Name, proto: probeIPv6})
 			}
 		}