Wait for the button to be clickable, and use it, instead of the ENTER key.

The previous implementation fails consistently for anonym, likely because the
ENTER key event isn't bound to its callback early enough, or similar. Let's hope
the OK button isn't displayed as being ready before it really is.

features/step_definitions/common_steps.rb

@@ -928,3 +928,8 @@ When /^I can print the current page as "([^"]+[.]pdf)" to the (default downloads
     @vm.file_exist?("#{output_dir}/#{output_file}")
   }
 end
+
+When /^I accept to import the key with Seahorse$/ do
+  next if @skip_steps_while_restoring_background
+  @screen.wait_and_click("TorBrowserOkButton.png", 10)
+end

features/torified_browsing.feature

@@ -30,7 +30,7 @@ Feature: Browsing the web using the Tor Browser
     And the Tor Browser has started and loaded the startup page
     And I open the address "https://tails.boum.org/tails-signing.key" in the Tor Browser
     Then I see "OpenWithImportKey.png" after at most 20 seconds
-    When I press the "ENTER" key
+    When I accept to import the key with Seahorse
     Then I see "KeyImportedNotification.png" after at most 10 seconds
 
   Scenario: Playing HTML5 audio