* Sockets: fix Sockets.killWorker not disconnecting connections
* Sockets: fix unit tests
- Fix crash when constructing mock sockets in certain cases
- Properly prevent workers from writing to stdout
- Fix race conditions in workers-related tests that were causing false
positives
* Tests: mock workers now more closely imitate sockets' workers
This helps catch cases where messages are being sent in the wrong order
to the workers, e.g. messages sent to sockets that no longer exist.
These tests are just sanity checks for sockets.js' exported methods.
Tests for bugs will come later when those for related users.js methods
have been written.