Fixes SocketMode support in slacktest

[prologic]

• Add support for /apps.conenctions.open endpoint for slacktest
• Add missing ping control message support for test server

Fixes #1244

[dnck]

Where's this PR stand? It's been open for quite some time, and I'm currently working on writing some tests against my app's usage of the websocket client, and I'm seeing similar issue as addressed in this open PR. I'll try it out the code changes and see how it goes.

[nlopes]

👋 I'm currently going over a review of most of the PRs here.

The only thing I'm not super keen in this PR is the introduction of go.mills.io/logger. I don't have an objection to the library per se but I'd rather not introduce something not updated in 3 years and already a fork of something that was archived.

@prologic what's your take here? Can we do something instead with logrus or the likes which are better maintained?

[dnck]

@nlopes that makes sense. If you're already using logrus, why introduce yet another logging framework!

[prologic]

Sorry folks, I've read these comments, but I haven't had the band with yet to respond. I'll have a look today if I have some time.