The rendering tests currently always save the impeller failure images into a temporary directory in /tmp which is out of the way and might accumulate over time.
The images are now only saved when `--save-impeller-failures` is specified on the command line and they are now saved into a local sub-directory with multiple runs saving into new sub-directories to keep the failure images from getting confused with each other over time. The new image directories are named `./impeller_failure_images/NNNN/*.png`