buffet: Re-enable buffet tests

Tests should work reliable with CL:272660

Following tests are re-enabled:
buffet_Registration
buffet_RefreshAccessToken
buffet_RestartWhenRegistered
buffet_IntermittentConnectivity
buffet_InvalidCredentials

BUG=brillo:990
TEST=test_that -b $BOARD $DEV_HOST e:buffet_.*

Change-Id: Ic3ae49dcdda3c5001a5e3297e8f926e5b0d7a987
Reviewed-on: https://chromium-review.googlesource.com/272740
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
6 files changed