Test suite: adjust Pidgin test to use a certificate that's still in Debian (refs: #15762).

ca-certificates 20161130+nmu1+deb9u1 removed the certificate we used
for testing.

features/step_definitions/pidgin.rb

@@ -456,7 +456,7 @@ end
 
 def pidgin_add_certificate_from (cert_file)
   # Here, we need a certificate that is not already in the NSS database
-  step "I copy \"/usr/share/ca-certificates/mozilla/CNNIC_ROOT.crt\" to \"#{cert_file}\" as user \"amnesia\""
+  step "I copy \"/usr/share/ca-certificates/mozilla/Staat_der_Nederlanden_EV_Root_CA.crt\" to \"#{cert_file}\" as user \"amnesia\""
 
   $vm.focus_window('Buddy List')
   @screen.wait_and_click('PidginToolsMenu.png', 10)