test: replace the disabled convex_hull_2d test (#14892)

test(libslic3r): replace the disabled convex_hull_2d test, closing #11269

The last "failing libslic3r test" from #11269 was the disabled
SCENARIO("2D convex hull of sinking object", "[3mf][.]") in test_3mf.cpp.
It checked ModelObject::convex_hull_2d for a sinking object against
PrusaSlicer's reference hull, but Orca's convex_hull_2d does not clip
geometry below the bed the way PrusaSlicer's its_convex_hull_2d_above does,
so the reference never matched. The test also wrote a debug mesh to a
hardcoded /tmp path and its comparison loop was inverted.

Remove it and add tests/libslic3r/test_model.cpp characterizing
convex_hull_2d on non-sinking transforms (identity and scale+offset),
where the projected footprint is unambiguous. Homed in a Model test file
since it exercises ModelObject, not 3MF.

tests/libslic3r/CMakeLists.txt

@@ -29,6 +29,7 @@ add_executable(${_TEST_NAME}_tests
     test_stl.cpp
     test_meshboolean.cpp
     test_marchingsquares.cpp
+    test_model.cpp
     test_utils.cpp
     test_timeutils.cpp
     test_voronoi.cpp