Resolve flaky double-clicking link test

Switch the test from a time-based waiting mechanism (1200 milliseconds)
to an event-based waiting mechanism.

```
2) [firefox] › navigation_tests.ts:341:1 › test double-clicking on a link ========================

  AssertionError: expected '/src/tests/fixtures/navigation.html' to equal '/__turbo/delayed_response'

    344 |
    345 |   await nextBody(page, 1200)
  > 346 |   assert.equal(pathname(page.url()), "/__turbo/delayed_response")
        |          ^
    347 |   assert.equal(await visitAction(page), "advance")
    348 | })
    349 |

      at /home/runner/work/turbo/turbo/src/tests/functional/navigation_tests.ts:346:10
```

src/tests/functional/navigation_tests.ts

@@ -339,10 +339,11 @@ test("test correct referrer header", async ({ page }) => {
 })
 
 test("test double-clicking on a link", async ({ page }) => {
-  page.click("#delayed-link")
-  page.click("#delayed-link")
+  await page.click("#delayed-link", { clickCount: 2 })
+
+  await nextEventNamed(page, "turbo:load")
+  await nextEventNamed(page, "turbo:load")
 
-  await nextBody(page, 1200)
   assert.equal(pathname(page.url()), "/__turbo/delayed_response")
   assert.equal(await visitAction(page), "advance")
 })