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_active_collection.py

@@ -0,0 +1,76 @@
+# ./blender.bin --background -noaudio --python tests/python/render_layer/test_active_collection.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 test_active_collection(self):
+        """
+        See if active collection index is working
+        layer.collections.active_index works recursively
+        """
+        import bpy
+        import os
+
+        ROOT = self.get_root()
+        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')
+        subzero.objects.link(three_b)
+        scorpion.objects.link(three_c)
+        layer = scene.render_layers.new('Fresh new Layer')
+        layer.collections.link(subzero)
+
+        lookup = [
+                'Master Collection',
+                '1',
+                'sub-zero',
+                'scorpion',
+                '2',
+                '3',
+                '4',
+                '5',
+                'sub-zero',
+                'scorpion']
+
+        for i, name in enumerate(lookup):
+            layer.collections.active_index = i
+            self.assertEqual(name, layer.collections.active.name,
+                    "Collection index mismatch: [{0}] : {1} != {2}".format(
+                        i, name, layer.collections.active.name))
+
+
+# ############################################################
+# 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()