ISO selection in test suite "Upgrade from ISO" is fragile

I’ve seen it fail on isotester1.lizard:

  Scenario: Upgrading an old Tails USB installation from an ISO image, running on the new version # features/usb_install.feature:234
    Given a computer                                                                              # features/step_definitions/common_steps.rb:66
    And I clone USB drive "old" to a new USB drive "to_upgrade"                                   # features/step_definitions/usb.rb:51
    And I setup a filesystem share containing the Tails ISO                                       # features/step_definitions/usb.rb:139
    And I start Tails from DVD with network unplugged and I login                                 # features/step_definitions/common_steps.rb:147
    And I plug USB drive "to_upgrade"                                                             # features/step_definitions/common_steps.rb:92
    And I do a "Upgrade from ISO" on USB drive "to_upgrade"                                       # features/step_definitions/usb.rb:144
      FindFailed: can not find USBCreateLiveUSB.png on the screen.

… so it seems that the code was already in usb_install_helper, while the previous steps (that select the ISO) have failed, as the attached screenshot shows. This should not happen.

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Parent Task: #8539 (closed)

Original created by @intrigeri on 9140 (Redmine)