Wait for HTTP/2 server to enable connect protocol

This is a consequence of updating Mint to 1.5.0+ - the SETTINGS frame is
now handled asynchronously (for the sake of latency) and Mint returns
default values until it arrives. So we need to wait for that frame to
arrive and be processed before upgrading an HTTP/2 WebSocket.

test/mint/web_socket_test.exs

@@ -119,10 +119,29 @@ defmodule Mint.WebSocketTest do
     end
   end
 
+  @doc !"""
+  In Mint 1.5.0+, Mint handles the SETTINGS frame from the server asynchronously
+  and returns default values for server settings until it is received. So we must
+  wait for the server to send the SETTINGS frame enabling the connect protocol.
+  """
+  defp wait_for_connect_protocol(conn) do
+    if HTTP2.get_server_setting(conn, :enable_connect_protocol) do
+      conn
+    else
+      receive do
+        message ->
+          {:ok, conn, []} = HTTP2.stream(conn, message)
+          wait_for_connect_protocol(conn)
+      end
+    end
+  end
+
   describe "given an HTTP/2 WebSocket connection to an echo server" do
     setup do
       {:ok, conn} = HTTP2.connect(:http, "localhost", 7070)
 
+      conn = wait_for_connect_protocol(conn)
+
       {:ok, conn, ref} =
         WebSocket.upgrade(:ws, conn, "/", [], extensions: [WebSocket.PerMessageDeflate])