test suite: fix shutdown step

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)

Original created by @tails on 5376 (Redmine)

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