all: use network less when running in v86 emulator

Updates #5794

Change-Id: I1d8b005a1696835c9062545f87b7bab643cfc44d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netcheck/netcheck.go

@@ -25,6 +25,7 @@ import (
 
 	"tailscale.com/derp/derphttp"
 	"tailscale.com/envknob"
+	"tailscale.com/hostinfo"
 	"tailscale.com/net/captivedetection"
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/neterror"
@@ -863,7 +864,7 @@ func (c *Client) GetReport(ctx context.Context, dm *tailcfg.DERPMap, opts *GetRe
 		c.curState = nil
 	}()
 
-	if runtime.GOOS == "js" || runtime.GOOS == "tamago" {
+	if runtime.GOOS == "js" || runtime.GOOS == "tamago" || (runtime.GOOS == "plan9" && hostinfo.IsInVM86()) {
 		if err := c.runHTTPOnlyChecks(ctx, last, rs, dm); err != nil {
 			return nil, err
 		}
@@ -1063,6 +1064,19 @@ func (c *Client) runHTTPOnlyChecks(ctx context.Context, last *Report, rs *report
 			regions = append(regions, dr)
 		}
 	}
+
+	if len(regions) == 1 && hostinfo.IsInVM86() {
+		// If we only have 1 region that's probably and we're in a
+		// network-limited v86 environment, don't actually probe it. Just fake
+		// some results.
+		rg := regions[0]
+		if len(rg.Nodes) > 0 {
+			node := rg.Nodes[0]
+			rs.addNodeLatency(node, netip.AddrPort{}, 999*time.Millisecond)
+			return nil
+		}
+	}
+
 	c.logf("running HTTP-only netcheck against %v regions", len(regions))
 
 	var wg sync.WaitGroup