Automated lint checking

[jml]

Recent discussions in #157 have highlighted the desirability of an automated lint checker with a configuration that's specialized for testtools' idiosyncratic needs.

This would be:

• part of the test suite
• configuration that developers could use for local & in-editor checks

If existing code doesn't meet the agreed-upon standard (and isn't that a discussion I'm looking forward to), then we can either ratchet the checks (i.e. no new lint) or have a flag day migration to the standard (there isn't that much WIP, or that much code).