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/kernel_other.go

@@ -0,0 +1,11 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package tstest
+
+// KernelVersion returns (0, 0, 0) on unsupported platforms.
+func KernelVersion() (major, minor, patch int) {
+	return 0, 0, 0
+}