also put INSERT_PRODUCTS_PATH in erp5testrunner's sys.path

Tests directory of inserted paths also needs to be in sys.path so that tests
can import other tests.

stack/erp5/buildout.cfg

@@ -388,6 +388,9 @@ initialization =
   sys.path[:0] = sum((
     glob.glob(os.path.join(x, 'Products', '*', 'tests'))
     for x in extra_path_list), [])
+  sys.path[:0] = sum((
+    glob.glob(os.path.join(x, 'Products', '*', 'tests'))
+    for x in os.getenv('INSERT_PRODUCTS_PATH', '').split(os.pathsep)), [])
 
 [test_suite_runner]
 # XXX: Workaround for fact ERP5Type is not an distribution and does not