tests: stabilize `test-extdiff.t` on macOS The recent change in the extdiff extension to take into account whether the GUI is accessible in d1b54c152673 started triggering this. I was able to run the test cleanly without this change at the console, but somewhere along the line, I read that the CI runner isn't able to access the GUI when not run as the root user. This is causing CI failures, so we conditionalize these tests out where `DISPLAY` is set to a non empty value to force `procutil.isgui()` to be True, when it in fact doesn't have GUI access.