add back the ability to skip reference file checks on some Coq versions
Alternative to !1009 (closed): just don't test reference files on 8.19 for now.
Having printing output reference files for multiple Coq versions would be possible, but that's a huge hassle. Iris contributors that change tests need to compile with multiple Coq versions to update both
.ref
files.
We had that infrastructure, but I agree it's not great, so I removed it.