Test suite: mark "Scenario: Tails shuts down on USB boot medium removal" as fragile.

... for the same reason as all scenarios that use Tails Installer.

refs: #10720

features/checks.feature

@@ -90,6 +90,8 @@ Feature: Various checks
     When I eject the boot medium
     Then Tails eventually shuts down
 
+  #10720
+  @fragile
   Scenario: Tails shuts down on USB boot medium removal
     Given I have started Tails without network from a USB drive without a persistent partition and logged in
     When I eject the boot medium