Investigate running tests on one runtime stack image at a time versus on all stacks at a time

[mhdawson]

As suggested in #4

This is a nice-to-have, but it would be good if we could run the tests just for one of the stacks at a time as well. If I'm making changes to the Node.js 18 image for example, I would appreciate being able to isolate the tests to that one stack. Open to having this implemented down the line though if you want.