Add "Test Alert" button when creating Slack alert schedule

**Is your feature request related to a problem? Please describe.**
Setting up Slack alert is easy, but when there is some integration problem, we would not know until the schedule passed.
And the issue must be troubleshooted through Metabase log by system admin.

**Describe the solution you'd like**
Why not adding a button to *Testing Alert* and display the error right away?

**Describe alternatives you've considered**
Wait and see the alert, then request system admin for the failure logs.

**How important is this feature to you?**
It is as important as Metabase alert, when it errors and we hardly able to troubleshoot, the feature is unusable.

**Additional context**
Also, I'm not sure if Slack integration support for Socket mode. It would be very useful if that so.

The issue names no files or tests. Start by locating the alert-schedule creation flow and Slack integration entry point; define done as a Test Alert control that reports integration failures immediately, with behavior for Socket mode clarified.

Written by the indexing model from the issue text.