Tests for implementations of IReactorFDSet.
| Function | socketpair | Undocumented |
| Class | ReactorFDSetTestsBuilder | Builder defining tests relating to IReactorFDSet. |
| Class | RemovingDescriptor | A read descriptor which removes itself from the reactor as soon as it
gets a chance to do a read and keeps track of when its own
fileno method is called. |