trees tests in multichrom branch need updating

[bhaller]

Broken trees tests in the multichrom branch need fixing. E.g.,

https://github.com/MesserLab/SLiM/actions/runs/11206337232/job/31146944105

Assigning to @petrelharp subsequent to Slack chat. It’d be nice to get that CI working again, especially since I’ll be actually starting to work on getting the tree-seq recording working for multiple chromosomes before too long. If you build SLiMgui from that branch, it ought to auto-correct most issues for you if you can open the test model in SLiMgui. (And I’ll be interested to hear about anything it fails to auto-correct! :->)