Test suite: fix the "Clone & Upgrade to a pristine USB drive" test to actually...

Test suite: fix the "Clone & Upgrade to a pristine USB drive" test to actually test what it's supposed to.

Otherwise it is just a clone of the same test for "Upgrade from ISO".

Bugfix against 7543c7d6, introduced
in Tails 1.5~rc1.

features/usb_install.feature

@@ -17,7 +17,7 @@ Feature: Installing Tails to a USB drive
     Given I have started Tails from DVD without network and logged in
     And I temporarily create a 4 GiB disk named "pristine"
     And I plug USB drive "pristine"
-    And I start Tails Installer in "Upgrade from ISO" mode
+    And I start Tails Installer in "Clone & Upgrade" mode
     Then a suitable USB device is not found
     And I am told that the destination device cannot be upgraded