Better TestWindow support (#3876)

Adding guest roles led us down a rabbit hole where we'd have liked to
rely on a
side-effect of activating a window in tests; but the test window didn't
implement that.

Looking into that, I realized our TestWindow wasn't doing a great job of
emulating the MacWindow, so this makes the two more similar.

crates/diagnostics/src/diagnostics.rs

@@ -1572,6 +1572,7 @@ mod tests {
             workspace::init_settings(cx);
             Project::init_settings(cx);
             crate::init(cx);
+            editor::init(cx);
         });
     }