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/waiterset_test.go

@@ -0,0 +1,46 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ipnserver
+
+import (
+	"context"
+	"sync"
+	"testing"
+)
+
+func TestWaiterSet(t *testing.T) {
+	var s waiterSet
+
+	wantLen := func(want int, when string) {
+		t.Helper()
+		if got := len(s); got != want {
+			t.Errorf("%s: len = %v; want %v", when, got, want)
+		}
+	}
+	wantLen(0, "initial")
+	var mu sync.Mutex
+	ctx, cancel := context.WithCancel(context.Background())
+
+	ready, cleanup := s.add(&mu, ctx)
+	wantLen(1, "after add")
+
+	select {
+	case <-ready:
+		t.Fatal("should not be ready")
+	default:
+	}
+	s.wakeAll()
+	<-ready
+
+	wantLen(1, "after fire")
+	cleanup()
+	wantLen(0, "after cleanup")
+
+	// And again but on an already-expired ctx.
+	cancel()
+	ready, cleanup = s.add(&mu, ctx)
+	<-ready // shouldn't block
+	cleanup()
+	wantLen(0, "at end")
+}