[client] categorize root/system-mutating tests behind a privileged build tag

Tests that need root or mutate host state (nftables/iptables/DNS, TUN/WireGuard
interfaces, routes, eBPF, SSH/service install) are now gated behind a
//go:build privileged tag. The default `go test ./client/...` runs as a non-root
user with no sudo and leaves host networking untouched; mixed files were split so
pure-logic tests stay in the default suite.

A self-hosting ory/dockertest/v4 harness (client/testutil/privileged) runs the
privileged suite inside a --privileged --cap-add=NET_ADMIN container via
`make test-privileged`; a DOCKER_CI=true guard skips the spawn when already inside
the container. Added `make test-unit` for the host-safe run.

client/firewall/iptables/manager_linux_test.go

@@ -1,3 +1,5 @@
+//go:build privileged
+
 package iptables
 
 import (

client/firewall/iptables/router_linux_test.go

@@ -1,4 +1,4 @@
-//go:build !android
+//go:build !android && privileged
 
 package iptables
 

client/firewall/nftables/manager_linux_test.go

@@ -1,3 +1,5 @@
+//go:build privileged
+
 package nftables
 
 import (

client/firewall/nftables/router_linux_test.go

@@ -1,4 +1,4 @@
-//go:build !android
+//go:build !android && privileged
 
 package nftables