Adds pharo-12 to test matrix

[lopezca]

[lberki]

PTAL! I'm becoming less and less sure that this is preferable to #22100 but at least it looks principle now (if complicated)

[lberki]

I think this would work, but @Wyverald decided to go with #22100 instead. I rate the two about equally in terms of complexity and probability that they fix the deadlock for good, so I think that's a reasonable decision.

@Wyverald : If #22100 doesn't work, let's take this pull request out from the closet. In the meantime, I'll close this since I hope it won't be necessary.