Fix stray :ssl message leak in HTTP/2 integration tests

The twitter.com describe block was using @moduletag instead of
@describetag, which made its connect: {"twitter.com", 443} tag apply
to the whole module. That caused the top-level setup to open an HTTPS
connection to twitter.com even for tests outside the describe (like
the TCP h2c test), and the SSL socket's SETTINGS frame would land in
the test mailbox. The previous test had a swallow-hack comment about
"SSL messages sneaking in"; this removes the root cause.

Author: ericmj

test/mint/http2/integration_test.exs

@@ -101,7 +101,7 @@ defmodule HTTP2.IntegrationTest do
   end
 
   describe "twitter.com" do
-    @moduletag connect: {"twitter.com", 443}
+    @describetag connect: {"twitter.com", 443}
     @browser_user_agent "Mozilla/5.0 (Macintosh; Intel Mac OS X 11_0_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36"
 
     test "ping", %{conn: conn} do