Bug 14402: Add unit tests for purge_zero_balance_fees()

Bug 14402: Add unit tests for purge_zero_balance_fees()

The function C4::Accounts::purge_zero_balance_fees() should delete rows
in accountlines where amountoutstanding is 0 and accountlines.date is
less than the current date minus '$days', i.e a number of days passed
to the function. Tests were added to prove the following:

* accountlines.amountoutstanding is 0, and date is set to CURRENT_DATE.
  The accountlines row should not be deleted. This is merely a sanity check,
  because difference between today's date and the fee date cannot be
  greater than $days.
* 'Before', 'On' and 'After' tests for accountlines.amountoutstanding = 0
    * accountlines.amountoutstanding is 0, and date is set to $days - 1
      days ago. The accountlines row should not be deleted.
    * accountlines.amountoutstanding is 0, and date is set to $days
      days ago.  the accountlines row should not be deleted, because
      the difference must be *greater* than $days.
    * accountlines.amountoutstanding is 0, and date is set to $days + 1
      days ago. The accountlines row *will* be deleted in this case.
* 'Before', 'On' and 'After' tests for accountlines.amountoutstanding > 0.
  (3 tests). The accountlines row will never be deleted, because
  amountoutstanding is not 0.
* 'Before', 'On' and 'After' tests for accountlines.amountoutstanding < 0.
  (3 testes) This tests credits. Again, the accountlines row will never
  be deleted, because amountoutstanding is not 0.

http://bugs.koha-community.org/show_bug.cgi?id=14402

Signed-off-by: Nancy Keener <nkeener@washoecounty.us>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>