buffet: Wrap unittest into buffet namespace

BUG=none
TEST=FEATURES=test emerge-gizmo buffet

Change-Id: I30af1e63b30ea273c40472fd6ed3caa3f1814ddd
Reviewed-on: https://chromium-review.googlesource.com/268314
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>