Testing for current date data

[kdorheim]

The R unit testing is pretty powerful but it does not check the values that are returned at the current state (if no date is provided, it assumes to return the current value/most recent date. This is how GCAM interacts with Hector). @pralitp and I ran into a problem caused by this when adding GMST to GCAM output. It would be good to test these outputs somehow may be with google tests?