[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.

docs/testing-privileged.md

@@ -0,0 +1,72 @@
+# Privileged tests
+
+Some tests in this repo need `root` or mutate host network state: they create
+TUN/WireGuard interfaces, open netlink/raw sockets, run eBPF programs, or shell
+out to `ip`/`iptables`/`nft`/`ifconfig`/`route`. Running them on a developer
+machine would require `sudo` and could leave stray interfaces or routes behind.
+
+These tests are gated behind the **`privileged` build tag** so the default test
+run is host-safe.
+
+## Running tests
+
+```bash
+# Host-safe: excludes privileged tests. Runs as a normal user, no sudo.
+make test-unit
+# equivalently:
+go test -tags devcert ./...
+
+# Privileged suite: runs the privileged-tagged tests inside a
+# --privileged --cap-add=NET_ADMIN container (requires Docker).
+make test-privileged
+```
+
+`make test-privileged` invokes the `ory/dockertest` harness in
+`client/testutil/privileged/`. The harness:
+
+1. Skips immediately when it detects it is already inside the container
+   (`DOCKER_CI=true`), so the privileged tests run in place instead of recursing.
+2. Otherwise spins up a `golang:1.25-alpine` container (matching CI),
+   bind-mounts the repo and the host Go build/module caches, installs the
+   required packages, and runs `go test -tags 'devcert privileged'` over the
+   client packages.
+3. Streams the container's output to the test log and fails if the suite fails.
+
+## Adding a privileged test
+
+A test is privileged if it does any of:
+
+- creates a real interface via `iface.NewWGIFace(...).Create()`,
+- opens a netlink or raw socket that hard-fails without `CAP_NET_ADMIN`,
+- runs an eBPF program (`ebpf.*.Listen()`),
+- shells out to `ip`, `iptables`, `nft`, `ifconfig`, or `route` to change state.
+
+Add the tag to the **top** of the file, combined with any existing platform
+constraint:
+
+```go
+//go:build privileged && linux
+
+package foo
+```
+
+If a file mixes privileged and pure-logic tests, **split it**: keep the pure
+tests (and any shared data — type/var declarations, table-driven `testCases`,
+helper interfaces) in an untagged file, and move the privileged tests into a
+`*_privileged_test.go` file with the tag. Shared declarations must stay untagged,
+otherwise the unprivileged files in the package will not compile.
+
+Always verify both build modes compile on every target platform:
+
+```bash
+go vet -tags devcert ./...
+go vet -tags 'devcert privileged' ./...
+```
+
+## CI
+
+- The `Client / Unit` job runs `go test -tags devcert` with **no** `sudo` — only
+  host-safe tests.
+- The `Client (Docker) / Unit` job runs `go test -tags 'devcert privileged'`
+  inside a `--privileged --cap-add=NET_ADMIN` container, which is where the
+  privileged tests actually execute.