shared: Fix test failures in BuxtonList
shared: Fix test failures in BuxtonList

BuxtonList was not accounting for size correctly in append and
prepend. BuxtonList also did not account for always keeping the tail
pointer valid. This change fixes these issues to pass tests.

Signed-off-by: William Douglas <07dd1d6cf12120bb0585d2eb50da402be35d56e5@intel.com>

sofar/buxton,sofar/buxton