Make the test suite more deterministic through network simulation

Originally created by @anonym on #9519 (Redmine)

As a Tails contributor
When Jenkins tells me that I broke something
Then I want to be able to trust it
And I won't ignore such notifications

This would eliminate all or parts of the issues we have with false test failures due to transient network errors.

Edited May 15, 2020 by anonym

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