No more need to alter sys.path in each and every render_layer unit test.

I tried the clean way, by setting the PYTHONPATH environment variable for
CTest, using SET (CTEST_ENVIRONMENT blablab), but that didn't seem to
work.

tests/python/render_layer/test_evaluation_visibility_a.py

@@ -6,7 +6,6 @@ import unittest
 import os
 import sys
 
-sys.path.append(os.path.dirname(__file__))
 from render_layer_common import *