accounting: fix test_roundDebitCredit_raises_if_big_difference

roundDebitCredit does not raise in case of big difference since
696bc0d2 (2009-02-06 yo * Stop raising an exception, even if the
credit is not equal to the debit in a built delivery, in the post
processing script, because the builder may not blindly adopt prevision
when updating existing lines, thus it can be unbalanced., 2009-02-06)

This assertRaises syntax does nothing on python2 and was an error on
python2. Update the test to test the actual behavior