Make it easier to update tst/worksheets/general.sheet/worksheet.g

[fingolfin]

Right now, when one edits, tst/worksheets/general.sheet/worksheet.g one then needs to "manually" update tst/worksheets/general.expected/. At the very least, we should document how to do this (e.g. via a comment inside tst/worksheets/general.sheet/worksheet.g). Perhaps we could also add a little script that does it automatically for the user (of course the changes made by that script then still need to be reviewed, but that's part of the regular PR review process anyway...)