Checks send no Referer header, so Hetzner answers (closes #191)

The Hetzner speed-test servers close the connection without an answer
when the request has a Referer header. The browser sends one unless the
page comes with the image's Referrer-Policy: no-referrer header, which a
static host and make dev do not send. Each check now sets
referrerPolicy: "no-referrer".

Model: opus-5-5

test/unit/main.test.js

@@ -180,6 +180,17 @@ test("a check fetches the target's URL as written, with no query string added",
     );
 });
 
+test("a check sends no Referer header, wherever the page is served from", async (t) => {
+    mockTargets(t, () => 10);
+    const check = measureLatency("https://fsn1-speed.hetzner.com");
+    t.mock.timers.tick(10);
+    assert.notEqual(await settled(check), "still waiting");
+    assert.equal(
+        fetch.mock.calls[0].arguments[1].referrerPolicy,
+        "no-referrer",
+    );
+});
+
 test("at a 30000ms interval, a target answering after 1000ms shows in its row while another target's check is still waiting", async (t) => {
     CONFIG.updateInterval = 30000;
     const state = new AppState([