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_layer_linking.py

@@ -0,0 +1,116 @@
+# ./blender.bin --background -noaudio --python tests/python/render_layer/test_layer_linking.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_layer_linking(self, filepath_json, link_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')
+
+            # 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')
+
+            # test linking sync
+            subzero.objects.link(three_b)
+            scorpion.objects.link(three_c)
+
+            # test unlinking sync
+            layer = scene.render_layers.new('Fresh new Layer')
+
+            if link_mode in {'COLLECTION_LINK', 'COLLECTION_UNLINK'}:
+                layer.collections.link(subzero)
+
+            if link_mode == 'COLLECTION_UNLINK':
+                initial_collection = layer.collections['Master Collection']
+                layer.collections.unlink(initial_collection)
+
+            # save file
+            filepath_nested = os.path.join(dirpath, 'nested.blend')
+            bpy.ops.wm.save_mainfile('EXEC_DEFAULT', filepath=filepath_nested)
+
+            # get the generated json
+            datas = query_scene(filepath_nested, 'Main', (get_scene_collections, get_layers))
+            self.assertTrue(datas, "Data is not valid")
+
+            filepath_nested_json = os.path.join(dirpath, "nested.json")
+            with open(filepath_nested_json, "w") as f:
+                for data in datas:
+                    f.write(dump(data))
+
+            self.assertTrue(compare_files(
+                filepath_nested_json,
+                filepath_json,
+                ),
+                "Scene dump files differ")
+
+    def test_syncing_layer_new(self):
+        """
+        See if the creation of new layers is going well
+        """
+        import os
+        ROOT = self.get_root()
+        filepath_json = os.path.join(ROOT, 'layers_new_layer.json')
+        self.do_layer_linking(filepath_json, 'LAYER_NEW')
+
+    def test_syncing_layer_collection_link(self):
+        """
+        See if the creation of new layers is going well
+        And linking a new scene collection in the layer works
+        """
+        import os
+        ROOT = self.get_root()
+        filepath_json = os.path.join(ROOT, 'layers_layer_collection_link.json')
+        self.do_layer_linking(filepath_json, 'COLLECTION_LINK')
+
+    def test_syncing_layer_collection_unlink(self):
+        """
+        See if the creation of new layers is going well
+        And unlinking the origin scene collection works
+        """
+        import os
+        ROOT = self.get_root()
+        filepath_json = os.path.join(ROOT, 'layers_layer_collection_unlink.json')
+        self.do_layer_linking(filepath_json, 'COLLECTION_UNLINK')
+
+
+# ############################################################
+# 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()