buffet: Wrap buffet unit tests into buffet namespace

Just to make referencing shorter by removing buffet:: prefix.

BUG=None
TEST=FEATURE=test emerge-gizmo buffet

Change-Id: Ie2cb5645911481c2eb92f80f4847b0faaaea532d
Reviewed-on: https://chromium-review.googlesource.com/268787
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>