Wait on turbo:load events during History visits

When navigating with `this.goBack()` and `this.goForward()`, synchronize
the timing by waiting on `turbo:load` events instead of the
`this.nextBody` hook firing.

During a restoration visit (triggered by `this.goBack()` or
`this.goForward()`), the [`turbo:render`][render] event will fire once
when restoring the cached snapshot, and a second time when the contents
are refreshed from a request to the server.

By waiting on the `turbo:load` event, the test won't proceed until that
process is complete, which will hopefully limit the possibility of
referencing stale elements.

[render]: https://turbo.hotwired.dev/reference/events

src/tests/functional/frame_tests.ts

@@ -380,7 +380,7 @@ export class FrameTests extends TurboDriveTestCase {
     await this.clickSelector("#link-frame")
     await this.nextEventNamed("turbo:load")
     await this.goBack()
-    await this.nextBody
+    await this.nextEventNamed("turbo:load")
 
     const title = await this.querySelector("h1")
     const frameTitle = await this.querySelector("#frame h2")
@@ -395,9 +395,9 @@ export class FrameTests extends TurboDriveTestCase {
     await this.clickSelector("#link-frame")
     await this.nextEventNamed("turbo:load")
     await this.goBack()
-    await this.nextBody
+    await this.nextEventNamed("turbo:load")
     await this.goForward()
-    await this.nextBody
+    await this.nextEventNamed("turbo:load")
 
     const title = await this.querySelector("h1")
     const frameTitle = await this.querySelector("#frame h2")