portlist,tstest: skip tests on kernels with /proc/net/tcp regression

Linux kernel versions 6.6.102-104 and 6.12.42-45 have a regression
in /proc/net/tcp that causes seek operations to fail with "illegal seek".
This breaks portlist tests on these kernels.

Add kernel version detection for Linux systems and a SkipOnKernelVersions
helper to tstest. Use it to skip affected portlist tests on the broken
kernel versions.

Thanks to philiptaron for the list of kernels with the issue and fix.

Updates #16966

Signed-off-by: Andrew Dunham <andrew@tailscale.com>

portlist/portlist_test.go

@@ -5,12 +5,24 @@ package portlist
 
 import (
 	"net"
+	"runtime"
 	"testing"
 
 	"tailscale.com/tstest"
 )
 
+func maybeSkip(t *testing.T) {
+	if runtime.GOOS == "linux" {
+		tstest.SkipOnKernelVersions(t,
+			"https://github.com/tailscale/tailscale/issues/16966",
+			"6.6.102", "6.6.103", "6.6.104",
+			"6.12.42", "6.12.43", "6.12.44", "6.12.45",
+		)
+	}
+}
+
 func TestGetList(t *testing.T) {
+	maybeSkip(t)
 	tstest.ResourceCheck(t)
 
 	var p Poller
@@ -25,6 +37,7 @@ func TestGetList(t *testing.T) {
 }
 
 func TestIgnoreLocallyBoundPorts(t *testing.T) {
+	maybeSkip(t)
 	tstest.ResourceCheck(t)
 
 	ln, err := net.Listen("tcp", "127.0.0.1:0")
@@ -47,6 +60,8 @@ func TestIgnoreLocallyBoundPorts(t *testing.T) {
 }
 
 func TestPoller(t *testing.T) {
+	maybeSkip(t)
+
 	var p Poller
 	p.IncludeLocalhost = true
 	get := func(t *testing.T) []Port {