Fixed unit tests.

src/libslic3r/GCode.cpp

@@ -1815,7 +1815,8 @@ namespace Skirt {
         // Extrude skirt at the print_z of the raft layers and normal object layers
         // not at the print_z of the interlaced support material layers.
         std::map<unsigned int, std::pair<size_t, size_t>> skirt_loops_per_extruder_out;
-        assert(skirt_done.empty());
+        //For sequential print, the following test may fail when extruding the 2nd and other objects.
+        // assert(skirt_done.empty());
         if (skirt_done.empty() && print.has_skirt() && ! print.skirt().entities.empty() && layer_tools.has_skirt) {
             skirt_loops_per_extruder_all_printing(print, layer_tools, skirt_loops_per_extruder_out);
             skirt_done.emplace_back(layer_tools.print_z);