hook_civicrm_managed leaves stale data

Steps:

1) Create a module or extension
2) Implement hook_civicrm_managed to define an entity 'foo'
3) Rebuild
4) Modify hook_civicrm_managed to remove entity 'foo'
5) Rebuild
6) Observe: 'foo' is still in the database

This is not supposed to happen – there was an oversight in the relevant unit-test which allowed this to go unnoticed.