add tests for writing records w/ DAGs

[wibeasley]

Daniela & Sebastian asked a question on the (private) REDCap forums about writing records with a DAG variable. I know we've done this in our projects, but was surprised that we don't have tests for it. (We have tests for writing and tests for DAGs, but not tests for the combination.)

I'll add some and see if this helps answer their question.

https://community.projectredcap.org/questions/114667/r-redcap-write-record-id-not-appearing-as-dag-reco.html

edit: also accommodate the scenario described by Joseph Perkins in
https://community.projectredcap.org/questions/115805/using-the-api-to-update-a-dag-code-runs-fine-excep.html