buffet: Split buffet_Registration into multiple tests.

We add four new tests.

 buffet_RestartWhenRegistered
 buffet_RefreshAccessToken
 buffet_InvalidCredentials
 buffet_IntermittentConnectivity

and factor all the common bits into a BuffetTester base class.

BUG=brillo:656
TEST=All five buffet_* tests pass.

Change-Id: I9fd69d68854e9699d05c5a7eb926027239656afa
Reviewed-on: https://chromium-review.googlesource.com/262910
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
11 files changed