Autopkgtest view should show last few lines of output from failing tests

This is a loose end from #162 (closed), which I'm re-filing separately so that we can close the original M1-targeted issue. The autopkgtest web view currently shows the status of each individual test, and allows downloading the full output via the generic artifact view. However, it should ideally also show some of the log output so that it's easier to see what happened.

When I mentioned this in a project meeting, Stefano pointed out that ci.debian.net recently gained a fancy log viewer with collapsible subsections. It would be useful to have something similar - perhaps we can even borrow the JavaScript more or less directly, with a bit of glue.