Following the pattern established by `Turbo.visit(...)` and
`Turbo.visit(..., { frame: "..." })`, return the `FrameElement.loaded`
promise from calls to `FrameElement.reload()`.

That way, callers can block on it:

```js
const frame = document.getElementById("my-frame")
await frame.reload()
// ...
```

Switch the tests from a time-based waiting mechanism (1200 milliseconds)
to an event-based waiting mechanism (waiting for `turbo:load`).

```
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

 1) [firefox] › navigation_tests.ts:131:1 › test following a same-origin POST form button[data-turbo-action=replace]

    page.click: Target closed
    =========================== logs ===========================
    waiting for selector "#same-origin-replace-form-submitter-post button"
      selector resolved to visible <button data-turbo-action="replace">Same-origin form[method="post"] button[data-turbo…</button>
    attempting click action
      waiting for element to be visible, enabled and stable
      element is visible, enabled and stable
      scrolling into view if needed
    ============================================================

      130 |
      131 | test("test following a same-origin POST form button[data-turbo-action=replace]", async ({ page }) => {
    > 132 |   page.click("#same-origin-replace-form-submitter-post button")
          |        ^
      133 |   await nextBody(page)
      134 |
      135 |   assert.equal(pathname(page.url()), "/src/tests/fixtures/one.html")

        at
/home/runner/work/turbo/turbo/src/tests/functional/navigation_tests.ts:132:8
```