testHandlingOfContributionInvoiceSetting() - More realistic data+assertion

testHandlingOfContributionInvoiceSetting() - More realistic data+assertion

In practice, for the old setting `contribution_invoice_settings`, the
`invoicing` property could take one of two values:

1. If enabled, the value is an array with trueish subkey: `['invoicing'=>['invoicing'=>1]]`
2. If disabled, the value is a numeric `0`: `['invoicing'=>0]`

Following this means better compatibility.

The format is counter-intuitive, and the unit-test sets the value to enabled
using a more intuitive format (`['invoicing'=>1]`).  It's currently valid to
`set()` this way, but `get()`ting the value produces the
more-compatible/historical format.