adds postephemeral handler to slacktest to audit outgoing messages

[mtintes]

The Changes

PostEphemeral was not exposing messages in the outgoing messages on slacktest. This is mostly a copy-paste from how postMessage handles that with the addition that we use the userId that gets handed into the method.

[mtintes]

Just wanted to check in on this to see if there were any questions or concerns surrounding it that I could help clear up. I kind of just threw this in because I needed it to write some tests and most of our sends are ephemeral messages.

[mtintes]

Hey just wanted to circle back and see if you had any thoughts on this? Happy to do the work if you have alternative ideas.

[mtintes]

hey @kanata2 I know it's been a while, but just want to follow up on this. Happy to update it and whatnot. Or if you have another idea I would happily work through it.