all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/integration_test.go

@@ -1673,7 +1673,7 @@ func TestNetstackTCPLoopback(t *testing.T) {
 		defer lis.Close()
 
 		writeFn := func(conn net.Conn) error {
-			for i := 0; i < writeBufIterations; i++ {
+			for range writeBufIterations {
 				toWrite := make([]byte, writeBufSize)
 				var wrote int
 				for {

tstest/integration/nat/nat_test.go

@@ -317,9 +317,7 @@ func (nt *natTest) runTest(addNode ...addNodeFunc) pingRoute {
 	}
 	defer srv.Close()
 
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
+	wg.Go(func() {
 		for {
 			c, err := srv.Accept()
 			if err != nil {
@@ -327,7 +325,7 @@ func (nt *natTest) runTest(addNode ...addNodeFunc) pingRoute {
 			}
 			go nt.vnet.ServeUnixConn(c.(*net.UnixConn), vnet.ProtocolQEMU)
 		}
-	}()
+	})
 
 	for i, node := range nodes {
 		disk := fmt.Sprintf("%s/node-%d.qcow2", nt.tempDir, i)
@@ -391,7 +389,6 @@ func (nt *natTest) runTest(addNode ...addNodeFunc) pingRoute {
 
 	var eg errgroup.Group
 	for i, c := range clients {
-		i, c := i, c
 		eg.Go(func() error {
 			node := nodes[i]
 			t.Logf("%v calling Status...", node)

tstest/integration/vms/vms_test.go

@@ -364,7 +364,7 @@ func (h *Harness) testDistro(t *testing.T, d Distro, ipm ipMapping) {
 		// starts with testcontrol sometimes there can be up to a few seconds where
 		// tailscaled is in an unknown state on these virtual machines. This exponential
 		// delay loop should delay long enough for tailscaled to be ready.
-		for count := 0; count < 10; count++ {
+		for range 10 {
 			sess := getSession(t, cli)
 
 			outp, err = sess.CombinedOutput("tailscale status")