Attempt to fix flaky Scroll Restoration test

There is a common test failure in CI:

```
× firefox on linux 5.11.0-1020-azure - ScrollRestorationTests - landing on an anchor (0.02s)
444
    TypeError: Cannot read property 'slice' of null
445
      at RemoteChannel.read @ src/tests/helpers/remote_channel.ts:14:44
```

This commit addresses it in two ways. First, ensures that the
`scroll_restoration.html` page loads the
`src/tests/fixtures/test.js` script to wire-up the remote channels (like
`window.eventLogs` and `window.mutationLogs`).

Next, it modifies the test itself to await the `this.nextBody` hook, to
ensure that the page load completes before checking the scroll.

src/tests/fixtures/scroll_restoration.html

@@ -4,6 +4,7 @@
     <meta charset="utf-8">
     <title>Scroll Restoration</title>
     <script src="/dist/turbo.es2017-umd.js" data-turbo-track="reload"></script>
+    <script src="/src/tests/fixtures/test.js"></script>
     <style>
       li {
         min-height: 50vh;

src/tests/functional/scroll_restoration_tests.ts

@@ -3,9 +3,8 @@ import { TurboDriveTestCase } from "../helpers/turbo_drive_test_case"
 export class ScrollRestorationTests extends TurboDriveTestCase {
   async "test landing on an anchor"() {
     await this.goToLocation("/src/tests/fixtures/scroll_restoration.html#three")
-    await this.nextBeat
+    await this.nextBody
     const { y: yAfterLoading } = await this.scrollPosition
-    await this.nextBeat
     this.assert.notEqual(yAfterLoading, 0)
   }