github: update PR template to require testing + make changelog notice seen by reviewers
Created by: slimsag
Fixes #2312 (closed)
I have intentionally made this short and concise. The point is to make both the author and reviewer think about updating the changelog and verifying the change is well tested, but not to be over-prescriptive (i.e., I want both the author and reviewer to think about these -- but not prescribe the exact course of action that needs to be taken).