test suite: usb install feature fails at shutdown time

I see:

And Tails is running from USB drive "current"                      # features/step_definitions/usb.rb:305
And the boot device has safe access rights                         # features/step_definitions/usb.rb:322
And I enable persistence with password "asdf"                      # features/step_definitions/usb.rb:248
And I log in to a new session                                      # features/step_definitions/common_steps.rb:165
And GNOME has started                                              # features/step_definitions/common_steps.rb:204
And I have closed all annoying notifications                       # features/step_definitions/common_steps.rb:254
And persistence has been enabled                                   # features/step_definitions/usb.rb:277
And I write some files expected to persist                         # features/step_definitions/usb.rb:354
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:86:in `And I completely shutdown Tails'
Scenario failed at time 00:46:52

And:

And the boot device has safe access rights                         # features/step_definitions/usb.rb:322
And I enable read-only persistence with password "asdf"            # features/step_definitions/usb.rb:271
And I log in to a new session                                      # features/step_definitions/common_steps.rb:165
And GNOME has started                                              # features/step_definitions/common_steps.rb:204
And I have closed all annoying notifications                       # features/step_definitions/common_steps.rb:254
And persistence has been enabled                                   # features/step_definitions/usb.rb:277
And I write some files not expected to persist                     # features/step_definitions/usb.rb:371
And I remove some files expected to persist                        # features/step_definitions/usb.rb:363
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:104:in `And I completely shutdown Tails'
Then only the expected files should persist on USB drive "current" # features/step_definitions/usb.rb:390
Scenario failed at time 00:56:00

Screenshots show a black screen.

Parent Task: #5978 (closed)

Original created by @tails on 5897 (Redmine)