Add turbo:before-prefetch test coverage

Expand the link prefetching functional test coverage to exercise
prefetching in relation to the events dispatched during the lifecycle.
For example, include coverage for:

* `turbo:before-prefetch` being dispatched
* `turbo:before-fetch-request` dispatched only once during a successful
  prefetch

src/tests/fixtures/hover_to_prefetch.html

@@ -4,6 +4,7 @@
     <meta charset="utf-8" />
     <title>Hover to Prefetch</title>
     <script src="/dist/turbo.es2017-umd.js" data-turbo-track="reload"></script>
+    <script src="/src/tests/fixtures/test.js"></script>
     <meta name="turbo-prefetch" content="true" />
   </head>
 

src/tests/fixtures/test.js

@@ -73,6 +73,7 @@
   "turbo:before-visit",
   "turbo:load",
   "turbo:render",
+  "turbo:before-prefetch",
   "turbo:before-fetch-request",
   "turbo:submit-start",
   "turbo:submit-end",

src/tests/functional/link_prefetch_observer_tests.js

@@ -1,6 +1,6 @@
-import { test } from "@playwright/test"
+import { expect, test } from "@playwright/test"
 import { assert } from "chai"
-import { nextBeat, sleep } from "../helpers/page"
+import { nextBeat, nextEventOnTarget, noNextEventOnTarget, sleep } from "../helpers/page"
 import fs from "fs"
 import path from "path"
 
@@ -17,7 +17,22 @@ test.afterEach(() => {
 
 test("it prefetches the page", async ({ page }) => {
   await goTo({ page, path: "/hover_to_prefetch.html" })
-  await assertPrefetchedOnHover({ page, selector: "#anchor_for_prefetch" })
+
+  const link = page.locator("#anchor_for_prefetch")
+
+  await link.hover()
+  await nextEventOnTarget(page, "anchor_for_prefetch", "turbo:before-prefetch")
+  const { url, fetchOptions } = await nextEventOnTarget(page, "anchor_for_prefetch", "turbo:before-fetch-request")
+
+  expect(url).toEqual(await link.evaluate(a => a.href))
+  expect(fetchOptions.headers["X-Sec-Purpose"]).toEqual("prefetch")
+
+  await link.hover()
+  await noNextEventOnTarget(page, "anchor_for_prefetch", "turbo:before-fetch-request")
+  await link.click()
+  await noNextEventOnTarget(page, "anchor_for_prefetch", "turbo:before-fetch-request")
+
+  await expect(page.locator("body")).toHaveText("Prefetched Page Content")
 })
 
 test("it doesn't follow the link", async ({ page }) => {