Update the old canvas test generator to minimise diff with new one.

This will make it easier to migrate old tests to the new test
generator. Without this CL, every single file will show as different.

Change-Id: If0468108530e756ebcbbf7e17e028b12ae31f57a
Bug: 1275750
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4543890


Reviewed-by: Yi Xu <yiyix@chromium.org>
Commit-Queue: Jean-Philippe Gravel <jpgravel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148138}