fix race conditions when upload progress redirects

[SteffenDE]

When someone does a push_navigate in the progress callback of an upload, the render_upload function of LiveViewTest sometimes did not properly return the live_redirect error tuple. This happened because the UploadChannel closes as soon as an upload is consumed. When this happens in the progress callback, the test code and the channel code raced:
The channel sends a redirect message to itself (or the root view) which shuts down once it receives it. The test code triggers a new render through the ClientProxy. Now it could happen that:

1. the test process can successfully call the ClientProxy, but the ClientProxy then crashed while pinging the channel
2. or the ClientProxy succeeds in pinging and actually returns the new HTML while the test expects the redirect error tuple
3. or the channel and client proxy shut down in time for the test process to fail rendering and return the error tuple as expected

We fix this race by ensuring that we ping the view (and root view) as soon as the upload is finished. This ensures that the channel has time to send the redirect message before the render call can ping it again. Thus, we always get the expected shutdown at the latest when the render call pings the channel.

Fixes #3662.
Relates to: #2047
Relates to: #1620

[SteffenDE]

this specifically prevents the error reported in the issue, as render_tree() can also return {:error, ...}

  5) test attempt: 1 redirects to the success page after a successful upload (DemoWeb.UploadLive.ShowTest)
     test/demo_web/live/upload_live/show_test.exs:8
     ** (FunctionClauseError) no function clause matching in Floki.RawHTML.build_raw_html/6

     The following arguments were given to Floki.RawHTML.build_raw_html/6:

         # 1
         [error: {:live_redirect, %{kind: :push, to: "/success"}}]

         # 2
         []

         # 3
         &Floki.Entities.encode/1

         # 4
         []

         # 5
         ["area", "base", "br", "col", "command", "embed", "hr", "img", "input", "keygen", "link", "menuitem", "meta", "param", "source", "track", "wbr"]

         # 6
         []

     Attempted function clauses (showing 6 out of 6):

         defp build_raw_html([], acc, _encoder, _pad, _self_closing_tags, _line_ending)
         defp build_raw_html([string | tail], acc, encoder, pad, self_closing_tags, line_ending) when is_binary(string)
         defp build_raw_html([{:comment, comment} | tail], acc, encoder, pad, self_closing_tags, line_ending)
         defp build_raw_html([{:pi, tag, attrs} | tail], acc, encoder, pad, self_closing_tags, line_ending)
         defp build_raw_html([{:doctype, type, public, system} | tail], acc, encoder, pad, self_closing_tags, line_ending)
         defp build_raw_html([{type, attrs, children} | tail], acc, encoder, pad, self_closing_tags, line_ending)

     code: html = render_upload(avatar, "kenya.jpg")
     stacktrace:
       (floki 0.37.0) lib/floki/raw_html.ex:73: Floki.RawHTML.build_raw_html/6
       (floki 0.37.0) lib/floki/raw_html.ex:68: Floki.RawHTML.raw_html/2
       (phoenix_live_view 1.0.4) lib/phoenix_live_view/test/live_view_test.ex:2024: Phoenix.LiveViewTest.render_chunk/3
       test/demo_web/live/upload_live/show_test.exs:22: (test)