atm: [he] adjust tests to account for sk_wmem_alloc changes

due to reference counting sk_wmem_alloc now has a value of 1 when all
the outstanding data has been sent.

Signed-off-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Signed-off-by: David S. Miller <davem@davemloft.net>