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/ipnserver/server_fortest.go

@@ -0,0 +1,42 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ipnserver
+
+import (
+	"context"
+	"net/http"
+
+	"tailscale.com/ipn/ipnauth"
+)
+
+// BlockWhileInUseByOtherForTest blocks while the actor can't connect to the server because
+// the server is in use by a different actor. It is used in tests only.
+func (s *Server) BlockWhileInUseByOtherForTest(ctx context.Context, actor ipnauth.Actor) error {
+	return s.blockWhileIdentityInUse(ctx, actor)
+}
+
+// BlockWhileInUseForTest blocks until the server becomes idle (no active requests),
+// or the specified context is done. It returns the context's error if it is done.
+// It is used in tests only.
+func (s *Server) BlockWhileInUseForTest(ctx context.Context) error {
+	ready, cleanup := s.zeroReqWaiter.add(&s.mu, ctx)
+
+	s.mu.Lock()
+	busy := len(s.activeReqs) != 0
+	s.mu.Unlock()
+
+	if busy {
+		<-ready
+	}
+	cleanup()
+	return ctx.Err()
+}
+
+// ServeHTTPForTest responds to a single LocalAPI HTTP request.
+// The request's context carries the actor that made the request
+// and can be created with [NewContextWithActorForTest].
+// It is used in tests only.
+func (s *Server) ServeHTTPForTest(w http.ResponseWriter, r *http.Request) {
+	s.serveHTTP(w, r)
+}