Correct revoke-sudo to actually work

* jenkins/jobs/macros.yaml(revoke-sudo): Simplify the sudoers
include file deletion to not rely on a conditional check, and then
test that it actually worked. Previously, systems where
/etc/sudoers.d was non-world-readable caused it to be a silent
no-op.

Change-Id: Ie713482acbd454eeb58c3481e8b8820049daaab8

jenkins/jobs/macros.yaml

@@ -15,10 +15,10 @@
     name: revoke-sudo
     builders:
       - shell: |
-          #!/bin/bash
-          if [ -f /etc/sudoers.d/jenkins-sudo ] ; then
-              sudo rm /etc/sudoers.d/jenkins-sudo
-          fi
+          #!/bin/bash -x
+          sudo rm -f /etc/sudoers.d/jenkins-sudo
+          # Prove that general sudo access is actually revoked
+          ! sudo -n true
 
 - builder:
     name: coverage