[OpenSCAD] Update STL unit test to warn on pipes

If OpenSCAD or FreeCAD is set up to use pipes for communication, assume
it's because one or the other is sandboxed. In that case, OpenSCAD will
not be able to import the generated STL file from FreeCAD, so don't try
to run that test.

src/Mod/OpenSCAD/OpenSCADTest/app/test_importCSG.py

@@ -305,6 +305,13 @@ polyhedron(
 #        FreeCAD.closeDocument(doc.Name)
 
     def test_import_import_stl(self):
+        preferences = FreeCAD.ParamGet("User parameter:BaseApp/Preferences/Mod/OpenSCAD")
+        transfer_mechanism = preferences.GetInt('transfermechanism',0)
+        if transfer_mechanism == 2:
+            print ("Cannot test STL import, communication with OpenSCAD is via pipes")
+            print ("If either OpenSCAD or FreeCAD are installed as sandboxed packages,")
+            print ("use of import is not possible.")
+            return
         testfile = join(self.test_dir, "Cube.stl").replace('\\','/')
         doc = self.utility_create_scad("import(\"{}\");".format(testfile), "import_stl");
         object = doc.ActiveObject