test: Split up and add tests for BuxtonList
shared: Set BuxtonList pointer to null after list free

Since we are being passed the address of the BuxtonList pointer, set
the pointer to NULL once buxton_list_free(_all) are finished.

Signed-off-by: William Douglas <william.douglas@intel.com>
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 <william.douglas@intel.com>