add helper to assert that a LiveView will receive a message

[SteffenDE]

Uses OTP 27's trace sessions to trace messages received by the LiveView.

cc @josevalim

Since this is not really LiveView specific, maybe better-suited in ExUnit?

[josevalim]

Yeah, I think it could be part of Elixir, but I am not sure I like the name "assert_will_receive". You may also want to receive multiple messages within a single trace/function. Maybe an API should be more like:

trace_messages(pid, fn ->
  do_something(...)
  assert_received {:trace, ^pid, :receive, {:msg}}
end)

We call it trace_messages if we want to allow opting into tracing send. Another option is to call it explicitly trace_receive (and we introduce trace_call and trace_send in the future). Thoughts?

[josevalim]

Or perhaps trace_receive(GenServer.whereis(), fn pid -> ... end).

[josevalim]

Another option is:

trace(pid, [:send, :receive, call: {m, f, a}], fn -> ... end)

But most usages would likely be:

trace(pid, [:receive], fn -> ... end)

That seems to be the one with most potential for growth.