arrow

ARROW-7550: [R][CI] Run donttest examples in CI

Following @khughitt 's fix for an example that didn't run (#6157), I wanted to make sure we fixed all examples and could prevent them from reoccurring. This adds `--run-donttest` a couple of places, including in CI, and fixes one other failure it uncovered.

Closes #6161 from nealrichardson/run-donttest and squashes the following commits:

b7b45c918 <Neal Richardson> Add --run-donttest a few places and fix examples

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>