ggplot2 is a big library with a bunch of dependencies,
making our already slow R tests even slower. This PR
switches them to using the tinytest library, which has
no dependencies and is much smaller.
- MRAN doesn't seem to have R 4.1 specific snapshots, so let's
default to RSPM for anything 4.1+.
- Otherwise, snapshot dates in 2022 will result in using rspm
We were doing this from an old MRAN snapshot. I moved the pin
a little ahead, so IRKernel can also be installed from CRAN
instead of from GitHub. R <= 4.0 gets the old version, and anything
newer gets a more recent version of devtools. This gives us
fast installs for IRkernel with binary packages.
Also add a R 4.0 and R 4.1 test
YuviPanda