wgengine/magicsock: ignore close races even harder.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -289,16 +289,12 @@ func meshStacks(logf logger.Logf, mutateNetmap func(idx int, nm *netmap.NetworkM
 			m.conn.UpdatePeers(peerSet)
 			wg, err := nmcfg.WGCfg(nm, logf, netmap.AllowSingleHosts, "")
 			if err != nil {
-				if ctx.Err() != nil {
-					// shutdown race, don't care.
-					return
-				}
 				// We're too far from the *testing.T to be graceful,
 				// blow up. Shouldn't happen anyway.
 				panic(fmt.Sprintf("failed to construct wgcfg from netmap: %v", err))
 			}
 			if err := m.Reconfig(wg); err != nil {
-				if ctx.Err() != nil {
+				if ctx.Err() != nil || errors.Is(err, errConnClosed) {
 					// shutdown race, don't care.
 					return
 				}