test: add HTML renderer verification fixture

render-test.html

@@ -0,0 +1,15 @@
+<!doctype html>
+<html>
+<head>
+  <title>Gitea renderer verification</title>
+  <style>.renderer-safe { color: red; }</style>
+</head>
+<body>
+  <h1 id="renderer-heading">Gitea HTML renderer works</h1>
+  <div class="renderer-safe" data-unsafe="must-be-removed">Benign rendered content</div>
+  <a class="renderer-link" href="https://example.com/" onclick="window.rendererEscaped=true">Safe link</a>
+  <a class="renderer-javascript" href="javascript:window.rendererEscaped=true">Unsafe link</a>
+  <img src="x" onerror="window.rendererEscaped=true" alt="unsafe-event-probe">
+  <script>window.rendererEscaped = true;</script>
+</body>
+</html>