ipn/ipn{server,test}: extract the LocalAPI test client and server into ipntest

In this PR, we extract the in-process LocalAPI client/server implementation from ipn/ipnserver/server_test.go
into a new ipntest package to be used in high‑level black‑box tests, such as those for the tailscale CLI.

Updates #15575

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/lapitest/backend.go

@@ -0,0 +1,63 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lapitest
+
+import (
+	"testing"
+
+	"tailscale.com/control/controlclient"
+	"tailscale.com/ipn/ipnlocal"
+	"tailscale.com/ipn/store/mem"
+	"tailscale.com/types/logid"
+	"tailscale.com/wgengine"
+)
+
+// NewBackend returns a new [ipnlocal.LocalBackend] for testing purposes.
+// It fails the test if the specified options are invalid or if the backend cannot be created.
+func NewBackend(tb testing.TB, opts ...Option) *ipnlocal.LocalBackend {
+	tb.Helper()
+	options, err := newOptions(tb, opts...)
+	if err != nil {
+		tb.Fatalf("NewBackend: %v", err)
+	}
+	return newBackend(options)
+}
+
+func newBackend(opts *options) *ipnlocal.LocalBackend {
+	tb := opts.TB()
+	tb.Helper()
+
+	sys := opts.Sys()
+	if _, ok := sys.StateStore.GetOK(); !ok {
+		sys.Set(&mem.Store{})
+	}
+
+	e, err := wgengine.NewFakeUserspaceEngine(opts.Logf(), sys.Set, sys.HealthTracker(), sys.UserMetricsRegistry(), sys.Bus.Get())
+	if err != nil {
+		opts.tb.Fatalf("NewFakeUserspaceEngine: %v", err)
+	}
+	tb.Cleanup(e.Close)
+	sys.Set(e)
+
+	b, err := ipnlocal.NewLocalBackend(opts.Logf(), logid.PublicID{}, sys, 0)
+	if err != nil {
+		tb.Fatalf("NewLocalBackend: %v", err)
+	}
+	tb.Cleanup(b.Shutdown)
+	b.DisablePortMapperForTest()
+	b.SetControlClientGetterForTesting(opts.MakeControlClient)
+	return b
+}
+
+// NewUnreachableControlClient is a [NewControlFn] that creates
+// a new [controlclient.Client] for an unreachable control server.
+func NewUnreachableControlClient(tb testing.TB, opts controlclient.Options) (controlclient.Client, error) {
+	tb.Helper()
+	opts.ServerURL = "https://127.0.0.1:1"
+	cc, err := controlclient.New(opts)
+	if err != nil {
+		tb.Fatal(err)
+	}
+	return cc, nil
+}