move the old assertion reinterpreting implementation to _assertionold.py

Also, seperate out some common code from the two.

--HG--
branch : trunk

py/code/testing/test_assertion.py

@@ -1,5 +1,5 @@
 import py
-from py.__.code._assertion import View
+from py.__.code._assertionold import View
 def exvalue():
     return py.std.sys.exc_info()[1]