Use six.moves.builtins to mock open in unit tests

This commit switches all the mocks for the open call to use the six
builtins module instead of __builtin__. This is needed because on
python 3 it was renamed to builtins, so to enable running the unit
tests on either environment we need to leverage the six module.

Change-Id: Idf8c0e9858451507c9ac0cfae747f90c6feb7f1e
2 files changed