Add test suite for autoreload extension

These were disabled because of interference with other tests, but this moves them to their own suite, so they'll be run in a separate process from other tests. It introduces the notion of 'special test suites', which don't correspond to a top-level package.

Closes #1912

At this point, iptest feels rather like a pile of hacks. At some point it could do with a substantial rework. But this gets those tests running again.