Layers: Unittesting - make them granular

Some tests may break Blender, which makes the entire unittest routine to fail.
They are now I isolate the tests into individual files

Kudos to Sybren Stüvel and Sergey Sharybin for the advice.

Note: at the moment test_link.py is failing (since a41bbfb7)

tests/python/render_layer/test_object_copy.py

@@ -0,0 +1,100 @@
+# ./blender.bin --background -noaudio --python tests/python/render_layer/test_object_copy.py -- --testdir="/data/lib/tests/"
+
+# ############################################################
+# Importing - Same For All Render Layer Tests
+# ############################################################
+
+import unittest
+
+import os, sys
+sys.path.append(os.path.dirname(__file__))
+
+from render_layer_common import *
+
+
+# ############################################################
+# Testing
+# ############################################################
+
+class UnitTesting(RenderLayerTesting):
+    def do_object_copy(self, mode):
+        import bpy
+        import os
+        import tempfile
+        import filecmp
+
+        ROOT = self.get_root()
+        with tempfile.TemporaryDirectory() as dirpath:
+            filepath_layers = os.path.join(ROOT, 'layers.blend')
+            filepath_json = os.path.join(ROOT, 'layers_object_copy_duplicate.json')
+
+            # open file
+            bpy.ops.wm.open_mainfile('EXEC_DEFAULT', filepath=filepath_layers)
+
+            # create sub-collections
+            three_b = bpy.data.objects.get('T.3b')
+            three_c = bpy.data.objects.get('T.3c')
+
+            scene = bpy.context.scene
+            subzero = scene.master_collection.collections['1'].collections.new('sub-zero')
+            scorpion = subzero.collections.new('scorpion')
+            subzero.objects.link(three_b)
+            scorpion.objects.link(three_c)
+            layer = scene.render_layers.new('Fresh new Layer')
+            layer.collections.link(subzero)
+
+            scene.render_layers.active_index = len(scene.render_layers) - 1
+
+            if mode == 'DUPLICATE':
+                # assuming the latest layer is the active layer
+                bpy.ops.object.select_all(action='DESELECT')
+                three_c.select_set(action='SELECT')
+                bpy.ops.object.duplicate()
+
+            elif mode == 'NAMED':
+                bpy.ops.object.add_named(name=three_c.name)
+
+            # save file
+            filepath_objects = os.path.join(dirpath, 'objects.blend')
+            bpy.ops.wm.save_mainfile('EXEC_DEFAULT', filepath=filepath_objects)
+
+            # get the generated json
+            datas = query_scene(filepath_objects, 'Main', (get_scene_collections, get_layers))
+            self.assertTrue(datas, "Data is not valid")
+
+            filepath_objects_json = os.path.join(dirpath, "objects.json")
+            with open(filepath_objects_json, "w") as f:
+                for data in datas:
+                    f.write(dump(data))
+
+            self.assertTrue(compare_files(
+                filepath_objects_json,
+                filepath_json,
+                ),
+                "Scene dump files differ")
+
+    def test_copy_object(self):
+        """
+        OBJECT_OT_duplicate
+        """
+        self.do_object_copy('DUPLICATE')
+
+    def test_copy_object_named(self):
+        """
+        OBJECT_OT_add_named
+        """
+        self.do_object_copy('NAMED')
+
+
+# ############################################################
+# Main - Same For All Render Layer Tests
+# ############################################################
+
+if __name__ == '__main__':
+    import sys
+
+    extra_arguments = sys.argv[sys.argv.index("--") + 1:] if "--" in sys.argv else []
+    sys.argv = [__file__] + (sys.argv[sys.argv.index("--") + 2:] if "--" in sys.argv else [])
+
+    UnitTesting._extra_arguments = extra_arguments
+    unittest.main()