ipn/ipnlocal: ensure TestServeUnixSocket actually serves a Unix socket

The test sets up an HTTP-over-Unix server and a reverse proxy pointed at
this server, but prior to this change did not round-trip anything to the
backing server. This change ensures that we test code paths which proxy
Unix sockets for serve.

Fixes #19232
Signed-off-by: Harry Harpham <harry@tailscale.com>

ipn/ipnlocal/serve_unix_test.go

@@ -8,6 +8,7 @@ package ipnlocal
 import (
 	"errors"
 	"fmt"
+	"io"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -101,6 +102,23 @@ func TestServeUnixSocket(t *testing.T) {
 	if rp.url.Host != "localhost" {
 		t.Errorf("url.Host = %q, want %q", rp.url.Host, "localhost")
 	}
+
+	req := httptest.NewRequest("GET", "http://foo.test.ts.net/", nil)
+	rec := httptest.NewRecorder()
+
+	rp.ServeHTTP(rec, req)
+	if rec.Code != http.StatusOK {
+		t.Fatal("unexpected response code:", rec.Code)
+	}
+	resp := rec.Result()
+	defer resp.Body.Close()
+	respB, err := io.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatal("read error:", err)
+	}
+	if string(respB) != testResponse {
+		t.Fatalf("unexpected response: want: '%s'; got: '%s'", testResponse, string(respB))
+	}
 }
 
 func TestServeUnixSocketErrors(t *testing.T) {