test: Update build test case to check prerendering

test/build.test.mjs

@@ -1,6 +1,8 @@
 import { execFile } from "node:child_process";
 import { test } from "node:test";
 import { promisify } from "node:util";
+import { promises as fs } from "node:fs";
+import assert from "node:assert";
 import { dir } from "./util.mjs";
 
 const execFileAsync = promisify(execFile);
@@ -12,4 +14,7 @@ test("builds demo successfully", async () => {
 		[dir("node_modules/vite/bin/vite.js"), "build"],
 		{ cwd: dir("demo"), encoding: "utf8" },
 	);
+
+	const outputHtml = await fs.readFile(dir("demo/dist/index.html"), "utf-8");
+	assert.match(outputHtml, /Hello from Preact/);
 });