- Incremental tests: only run tests that were updated [idea]

  If a test is more than an executable, then tracking its "inputs" (so that we
  know whether to run it) will be tricky. Such a test will most likely run via
  testscript so can we capture inputs somehow? This seems to be related to the
  question of what to attach testscripts to. Maybe we should make them (in
  source) targets?

  Will also need some kind of switch/override to run all tests (command line
  config override seems like a natural choice).