Fix failing tests from changes in R 4.0

R 4.0 changes the default conversion to factors for data.frame:

> R now uses a stringsAsFactors = FALSE default, and hence by default no longer converts strings to factors in calls to data.frame() and read.table().

Our test previously verified the old default of converting to factors.
Now we confirm that the default is NOT a factor conversion, and that we
can explicitly change stringsAsFactors to get it to convert as it did
before.