... but running the Stem tests to learn about Tor code would not be helpful.
I have mixed feelings on this. Stem's tests are chiefly to test Stem, but the integration tests are for checking how our library interacts with a live Tor instance. There's a lot of value to Stem for us to prevent bugs and regression in the Tor controller interface - it helps Stem's users and prevents us from needing to implement gross hacks (see for instance the connection.py code for handling #4881).
I'd love for Tor developers to expand Stem's integ tests to better test's Tor's controller interface. I'm not, however, interested in having Stem test Tor's other functionality (relaying, hidden service, etc). I very much agree that down that road lies terrible scope creep.
Cheers! -Damian