Adding unit tests to validate individual components.

Testing make_perimeters() on a cube.
Also added simple test for make_skirt on a single cube.

src/test/libslic3r/test_print.cpp

@@ -0,0 +1,60 @@
+#include <catch.hpp>
+#include <string>
+#include "test_data.hpp"
+#include "libslic3r.h"
+
+using namespace Slic3r::Test;
+using namespace std::literals;
+
+SCENARIO("PrintObject: Perimeter generation") {
+    GIVEN("20mm cube and default config") {
+        auto config {Slic3r::Config::new_from_defaults()};
+        TestMesh m { TestMesh::cube_20x20x20 };
+        Slic3r::Model model;
+        auto event_counter {0U};
+        std::string stage;
+        int value {0};
+        auto callback {[&event_counter, &stage, &value] (int a, const char* b) { stage = std::string(b); event_counter++; value = a; }};
+        config->set("fill_density", 0);
+
+        WHEN("make_perimeters() is called")  {
+            auto print {Slic3r::Test::init_print({m}, model, config)};
+            const auto& object = *(print->objects.at(0));
+            print->objects[0]->make_perimeters();
+            THEN("67 layers exist in the model") {
+                REQUIRE(object.layers.size() == 67);
+            }
+            THEN("Every layer in region 0 has 1 island of perimeters") {
+                for(auto* layer : object.layers) {
+                    REQUIRE(layer->regions[0]->perimeters.size() == 1);
+                }
+            }
+            THEN("Every layer in region 0 has 3 paths in its perimeters list.") {
+                for(auto* layer : object.layers) {
+                    REQUIRE(layer->regions[0]->perimeters.items_count() == 3);
+                }
+            }
+        }
+    }
+}
+SCENARIO("Print: Skirt generation") {
+    GIVEN("20mm cube and default config") {
+        auto config {Slic3r::Config::new_from_defaults()};
+        TestMesh m { TestMesh::cube_20x20x20 };
+        Slic3r::Model model;
+        auto event_counter {0U};
+        std::string stage;
+        int value {0};
+        config->set("skirt_height", 1);
+        config->set("skirt_distance", 1);
+        WHEN("Skirts is set to 2 loops")  {
+            config->set("skirts", 2);
+            auto print {Slic3r::Test::init_print({m}, model, config)};
+            print->make_skirt();
+            THEN("Skirt Extrusion collection has 2 loops in it") {
+                REQUIRE(print->skirt.items_count() == 2);
+                REQUIRE(print->skirt.flatten().entities.size() == 2);
+            }
+        }
+    }
+}