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_link.py

@@ -0,0 +1,55 @@
+# ./blender.bin --background -noaudio --python tests/python/render_layer/test_link.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_link(self, master_collection):
+        import bpy
+        self.assertEqual(master_collection.name, "Master Collection")
+        self.assertEqual(master_collection, bpy.context.scene.master_collection)
+        master_collection.objects.link(bpy.data.objects.new('object', None))
+
+    def test_link_scene(self):
+        """
+        See if we can link objects
+        """
+        import bpy
+        master_collection = bpy.context.scene.master_collection
+        self.do_link(master_collection)
+
+    def test_link_context(self):
+        """
+        See if we can link objects via bpy.context.scene_collection
+        """
+        import bpy
+        bpy.context.scene.render_layers.active_index = len(bpy.context.scene.render_layers) - 1
+        master_collection = bpy.context.scene_collection
+        self.do_link(master_collection)
+
+
+# ############################################################
+# 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()