tstest/natlab/vmtest: add macOS VM screenshot streaming to web UI

When --vmtest-web is set, Host.app is launched with --screenshot-port 0
to start a localhost HTTP server that captures the VZVirtualMachineView
display. The Go test harness parses the SCREENSHOT_PORT=<port> line from
stdout, then polls every 2 seconds for JPEG thumbnails and pushes them
over WebSocket to the web dashboard.

Clicking a screenshot thumbnail opens a full-resolution image proxied
through the web UI's /screenshot/{node} endpoint.

Screenshot events are excluded from the EventBus history (they're large
and only the latest matters, stored in NodeStatus.Screenshot).

Updates #13038

Change-Id: I9bc67ddd1cc72948b33c555d4be3d8db06a41f6d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/natlab/vmtest/assets/index.html

@@ -47,6 +47,7 @@
             </div>
             {{end}}
         </div>
+        <div class="screenshot" id="screenshot-{{$node.Name}}">{{if $node.Screenshot}}<a href="/screenshot/{{$node.Name}}" target="_blank"><img src="{{$node.Screenshot}}"></a>{{end}}</div>
         <div class="console" id="console-{{$node.Name}}">{{range $node.Console}}{{ansi .}}
 {{end}}</div>
     </div>