Add some test-cases for gnc_numerics using the reciprocal denominator.
Even though we don't explicitly ask for this representation, certain
calculations (e.g. those involving GNC_HOW_DENOM_SIGFIGS in the loan
druid and soon in budgets) will return numbers using this representation.
Unfortunately, these numbers aren't handled correctly by most of the
gnc_numeric functions, like the ones for comparison, equality testing,
arithmetic and conversion.