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/style.css

@@ -120,6 +120,20 @@ h2 {
     color: #4af;
 }
 
+/* VM display screenshot */
+.screenshot:empty { display: none; }
+.screenshot {
+    margin-bottom: 4px;
+}
+.screenshot img {
+    width: 100%;
+    height: auto;
+    display: block;
+    border-radius: 4px;
+    border: 1px solid #222;
+    cursor: pointer;
+}
+
 /* Console output */
 .console {
     background: #0a0a0a;