Add Sweepers for test cleanup

[jharley]

The test suite is still leaving many things behind after a test run: notably Recipients even after a fully successful run. This gets extra sticky if a test run is unable to clean up (panic, cancelled early) and dangling resources end up causing conflicts on the following test runs.

Implementing Sweepers may help ease the pain here, especially if the entire test suite ends up prefixing resources with test- or test. or some such thing.