Fix for CGAL Error

[jeroenrnl]

Fixes the following error when running recent OpenSCAD versions

WARNING: CGAL error in CGAL_Nef_polyhedron3(): CGAL ERROR: assertion violation!
Expr: e_below != SHalfedge_handle()
File: /usr/include/CGAL/Nef_3/SNC_FM_decorator.h
Line: 418. Attempting union...
WARNING: CGALUtils::convertSurfaceMeshToNef: Discarded 80 facets during Nef conversion.

Now the tests.py runs with the git version of OpenSCAD, with only offset.scad and round.scad failing with the error

WARNING: [manifold] Minkowski failed with error, falling back to Nef operation: bad any_cast

This is openscad/openscad#6083 / openscad/openscad#5862
and these are only warnings, the actual result still works.

If I temporary sabotage openscad.py and bom.py to not fail on warnings, the tests run and the resulting png is virtually identical to the ones currently in git.