Test suite: add "Emergency shutdown" feature description.
|Feature: Emergency shutdown|
|As a Tails user|
|when I unplug my Tails device to trigger emergency shutdown|
|I want the system memory to be free from sensitive data.|
|# Test something close to real-world usage, without interfering,|
|# i.e. without the "I prepare Tails for memory erasure tests" step;|