I am looking for a quantification of my knowledge of college subjects that I could put on a resume, and preferably something that is free. It could supplement my course grades, and especially so if I continued to maintain that knowledge or never took that particular course. Thank you for your help!

There are the CLEP exams on Collegeboard, but they cost $80. https://clep.collegeboard.org/exams

Regarding computer science, I would think that the Linux community, being open-source, would have plenty of people willing to create free Linux tests.

Not quite what you sought but how about building a portfolio of work? Depends on the subjects you have studied, but something like coding up a statistical analysis on some free data set and putting it up on something like github. Apparently potential employers will look at people's github accounts. I have a friend who created a medical app for iphones - he just takes it to interviews and shows them what he's done, rather than just listing it.