Description
I previously did this in #33613, but lacking review, this never landed. Since then, the two scripts have been substantially modified (https://github.com/web-platform-tests/wpt/commits/master/html/canvas/tools/gentestutils.py, https://github.com/web-platform-tests/wpt/commits/master/html/canvas/tools/gentestutilsunion.py).
I'm slightly surprised/saddened at the size of cc1bc6f (from #40176), modifying the one build script to make the output between the two scripts more similar, and changing 1502 generated files, given my old PR managed to merge the two scripts with only 138 generated files changed in total. Maybe by the point that it landed it was impossible to have a smaller diff, but it still feels very unfortunate that we went from merging the scripts being something that was done and didn't change many generated files at all, to something where even reducing the diff between the two scripts has involved changing an order of magnitude more generated files.
cc @graveljp, who seems to have done most of the updates since my early 2022 PR.