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>

tstest/tstest_test.go

@@ -3,7 +3,10 @@
 
 package tstest
 
-import "testing"
+import (
+	"runtime"
+	"testing"
+)
 
 func TestReplace(t *testing.T) {
 	before := "before"
@@ -22,3 +25,17 @@ func TestReplace(t *testing.T) {
 		t.Errorf("before = %q; want %q", before, "before")
 	}
 }
+
+func TestKernelVersion(t *testing.T) {
+	switch runtime.GOOS {
+	case "linux":
+	default:
+		t.Skipf("skipping test on %s", runtime.GOOS)
+	}
+
+	major, minor, patch := KernelVersion()
+	if major == 0 && minor == 0 && patch == 0 {
+		t.Fatal("KernelVersion returned (0, 0, 0); expected valid version")
+	}
+	t.Logf("Kernel version: %d.%d.%d", major, minor, patch)
+}