Clean up splitintput() example output

[dchud]

Eight df.0000[0-9].csv files linger after running the splitinput() example, which causes a CHECK note as they are "non-standard things in the check directory". But if they are explicitly removed at the end of the example, a CHECK error occurs. with a file missing.

[dchud]

Per https://community.rstudio.com/t/r-cmd-check-r-4-0-0-now-runs-donttest-how-to-proceed-with-long-running-examples/64493/4, it appears the solution is to change donttest to dontrun on the splitinput() examples.