I think, what @nulltoken meant to say is: to explicitly test that after regisration/unregistration, it is really unregistered (thus cannot be unregistered again _after it was registered once_). This would fail, if i.e. the `_unregister` suddenly forgets to remove the transport from a table.
But looking at the code more deeply, it seems as this is implicitly tested by the combination of the 3 tests (esp. their order). i.e.: The 3rd test-pair implicitly tests a) that if `foo` was registered once, it can still be registered after it has been unregistered and b) that `foo` can be unregistered as long as it is registered (this is redundant inside this test code, actually).