Bug 21720: Update Tests

Bug 21720: Update Tests

AddIssuing charge is always called in void context, but the tests
checked that the return value was a Koha::Account::Offset and we now
return a Koha::Account::Line.

This patch changes the test to accept a Koha::Account::Line as returned,
adds a test to ensure a Koha::Account::Offset is also created and
corrects the logic that tests that only one accountline is added.

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>