Automerge: [Github] Add workflow to test the issue write workflow

This does not test most of the functionality (i.e., that writing to an
existing comment still works), but does ensure that the plumbing works
and things are not completely broken.

Reviewers: tstellar, cmtice

Reviewed By: cmtice

Pull Request: llvm/llvm-project#170209

Author: boomanaiden154