Remove sleep() from tests

[jku]

Description of issue or feature request:

Tests use sleep() quite a lot: this makes them slow, and they still don't always protect whatever the sleep is trying to protect.

Current behavior:

• python3 test_updater.py takes 25 seconds to run 38 tests
• Running the same command immediately again often fails (the sockets are still in TIME-WAIT state for a minute or so)
• Sometimes subprocess killing fails silently

Status:
I have a work-in-progress branch: https://github.com/jku/tuf/tree/remove-sleep-wip : it might not take care of all corner cases yet, the comments are still about the old implementation and it only covers test_updater.py but it does take execution time of python3 test_updater.py from 25 seconds to 8 seconds.

I'm filing this so I don't forget the branch on the holiday -- but 🥇 for anyone who takes it, polishes and extends it

[jku]

Fixed in pr #1096