To be honest, it was just a few minutes of searching "test print" or "calibration" on Thingiverse.
What I used on campus and at work were the 3D Kitbash test chips, very complete and very thorough (and they look good enough to display with your printer) but they aren't free. When I have my own printer, I might buy the 3DKB Graphica pack to print for the bragging rights, but those are specifically intended to be challenging.

I'm one of those very strange people who will actually pay money for a 3D model if I like it enough, and I recognize I'm in the minority there.