fix: Use sync.WaitGroup to await hijacked HTTP connections (#337)

WebSockets hijack the HTTP connection from the server, causing
server.Close() to not wait for these connections to fully cleanup.

This adds a global wait-group to the coderd API, which ensures all
WebSocket HTTP handlers have properly exited before returning.

coderd/coderdtest/coderdtest.go

@@ -55,7 +55,7 @@ func New(t *testing.T) *codersdk.Client {
 		})
 	}
 
-	handler := coderd.New(&coderd.Options{
+	handler, closeWait := coderd.New(&coderd.Options{
 		Logger:   slogtest.Make(t, nil).Leveled(slog.LevelDebug),
 		Database: db,
 		Pubsub:   pubsub,
@@ -69,7 +69,10 @@ func New(t *testing.T) *codersdk.Client {
 	srv.Start()
 	serverURL, err := url.Parse(srv.URL)
 	require.NoError(t, err)
-	t.Cleanup(srv.Close)
+	t.Cleanup(func() {
+		srv.Close()
+		closeWait()
+	})
 
 	return codersdk.New(serverURL)
 }