Test fix - assertion fails when run with other tests (sometimes)

Test fix - assertion fails when run with other tests (sometimes)

The assertion that invoice_number = INV_ + contribution_id is wrong. In fact
it is calculated as INV_ + (MAX(contribution_id)+1) prior to the contribution
being saved. This means that if the table has not been truncated the contribution
id might be 3 but the invoice_number would still be 0+1 - ie INV_1

- we could argue about whether the calculation method is flawed but this at least
makes the test more reliable and no-one has complained in practice about the
code behaviour