Fix the flushing of messages when switching to Websocket

We now flush messages that are specific to cowboy_http only.

Stream handlers should also flush their own specific messages
if necessary, although timeouts will be flushed regardless
of where they originate from.

Also renames the http_SUITE to old_http_SUITE to distinguish
new tests from old tests. Most old tests need to be removed
or converted eventually as they're legacy tests from Cowboy 1.0.

test/old_http_SUITE_data/http_errors.erl

@@ -0,0 +1,17 @@
+%% Feel free to use, reuse and abuse the code in this file.
+
+-module(http_errors).
+
+-export([init/2]).
+
+init(Req, _Opts) ->
+	#{'case' := Case} = cowboy_req:match_qs(['case'], Req),
+    case_init(Case, Req).
+
+case_init(<<"init_before_reply">> = Case, _Req) ->
+	ct_helper_error_h:ignore(?MODULE, case_init, 2),
+    error(Case);
+case_init(<<"init_after_reply">> = Case, Req) ->
+	ct_helper_error_h:ignore(?MODULE, case_init, 2),
+    _ = cowboy_req:reply(200, #{}, "http_handler_crashes", Req),
+    error(Case).