Publish testcraft from PRs to relevant branches

### What needs to get done

Once we have completed https://github.com/canonical/craft-actions/issues/48

Use that action to publish the testcraft snap to the `latest/edge/craft-application-` branch.

Running on main should publish to the `latest/edge/craft-application-main` branch. NOTHING should release directly to any public channels.

When running from a PR, this workflow should also make a comment notifying that the snap has been released. This should include both `snap install` and `snapcraft refresh --amend` commands for easy copy/paste.

### Why it needs to get done

Make it far easier for reviewers to test changes in a real-world situation.

First read craft-actions issue 48, then inspect the repository's workflow configuration for the testcraft publishing entry point; the issue names no specific file or test. Done means main publishes only to latest/edge/craft-application-main, PRs publish only to their numbered branches, no public channel is used, and PR comments include both requested commands.

Written by the indexing model from the issue text.