Fix SocketMode support in slacktest

[prologic]

          Hey 👋 Did the [slacktest](https://pkg.go.dev/github.com/slack-go/slack/slacktest) package ever get updated at all? Doesn't look like it did by this PP, and that means using it to unit test your bot's business logic doesn't currently work :/ e.g:

time="2023-12-11T14:14:58+10:00" level=debug msg="Starting SocketMode" file=socketmode/socket_mode_managed_conn.go line=293 logger=log2Logrus
2023/12/11 14:14:58 (127.0.0.1:57589 -) "POST /apps.connections.open HTTP/1.1" 404 19 41µs
time="2023-12-11T14:14:58+10:00" level=debug msg="HTTP/1.1 404 Not Found\r\nContent-Length: 19\r\nContent-Type: text/plain; charset=utf-8\r\nDate: Mon, 11 Dec 2023 04:14:58 GMT\r\nX-Content-Type-Options: nosniff\r\n\r\n404 page not found\n\n" file=misc.go line=266 logger=log2Logrus

Originally posted by @prologic in #885 (comment)