Run Windmill tests by default

Windmill tests should be part of the regular test suite. They are currently excluded; you have to run them explicitly, and we have a jscheck buildbot builder that runs those tests. This works badly, though, since we don't enter testfix mode on test failures, so the test suite is allowed to fail for a long time, making it harder to catch regressions.

The lp and db_lp buildbot builders should run the Windmill tests. When this is done, we can get rid of the jscheck builder.