test suite: fix shutdown step
Originally created by Tails on #5376 (Redmine)
In the "Writing files to a read-only-enabled persistent partition" scenario:
And I completely shutdown Tails # features/step_definitions/common_steps.rb:402 The image 'TailsEmergencyShutdownHalt.png' did not match in this region. (Sikuli::ImageNotFound) ./features/support/helpers/sikuli_helper.rb:31:in `wait_and_click' ./features/step_definitions/common_steps.rb:407:in `(root)':in `/^I completely shutdown Tails$/' features/usb_install.feature:103:in `And I completely shutdown Tails'
Parent Task: #5978 (closed)