Create tests for oversize images

Images can be too large for one of two reasons, and we could consider making tests to identify in either case:

Disk size too large

As encountered in #281031 (closed), we can see the image sizes with this command:

du -ah . | grep -v "/$" | grep ".png" | sort -rh

We could take this a step or two further, set a ceiling, and fail a test if an image is too large on disk.

Pixel dimensions too large

As found in this page the following command returns a list of all images that are larger than 3000px on a side:

find . -name '*.png' -exec file {} \; | sed 's/\(.*png\): .* \([0-9]* x [0-9]*\).*/\2 \1/' | awk 'int($1) > 3000 {print}'

We have a soft limit, but we might want to consider a sanity-check limit … we currently have an 10484 x 816 image (doc/development/img/memory_ruby_heap_fragmentation.png)

Related to #281031 (closed)