FEM: gmsh tools, use fixed order for group data in geo file

it is needed to make a unit test for geo writing

src/Mod/Fem/FemGmshTools.py

@@ -430,7 +430,7 @@ class FemGmshTools():
             # print('  We are going to have to find elements to make mesh groups for.')
             geo.write("// group data\n")
             # we use the element name of FreeCAD which starts with 1 (example: 'Face1'), same as GMSH
-            for group in self.group_elements:
+            for group in sorted(self.group_elements.keys()):  # for unit test we need them to have a fixed order
                 gdata = self.group_elements[group]
                 # print(gdata)
                 # geo.write("// " + group + "\n")