CRM-20750 follow up fix, reversal should be against original account.

CRM-20750 follow up fix, reversal should be against original account.

On trying to investigate why a test change was required in the original PR I came to the following conclusions:

1) the test function was causing problems by being hard to read. As a result it was too easy to see it as
'accounts magic' - I started a cleanup here
2) Once I understood what was being tested & failing I concluded the change in the PR was in fact wrong
as it was putting a negative entry against the new financial account rather than the original one.