add /open command (#6717)

api/command_open_test.go

@@ -0,0 +1,12 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package api
+
+import (
+	"testing"
+)
+
+func TestOpenCommands(t *testing.T) {
+	testJoinCommands(t, "open")
+}