buffet: fix the unit test. Fix the unit test. refresh Access Token does not generate Invalid Credentials any more. BUG=b:38308727 TEST=unit test. Change-Id: Ibac59cfdc203f1dd92f9371ede2742503d12b021 Reviewed-on: https://weave-review.googlesource.com/10010 Reviewed-by: Kan Yan <[email protected]>