Add test.ping to sudo_salt_safe

Allow users associated with sudo_salt_safe to execute test.ping for better visibility.
diff --git a/linux/system/sudo.yml b/linux/system/sudo.yml
index a2c7822..6e2d0af 100644
--- a/linux/system/sudo.yml
+++ b/linux/system/sudo.yml
@@ -29,6 +29,7 @@
     - /usr/bin/salt * pillar*
     - /usr/bin/salt * grains*
     - /usr/bin/salt * saltutil*
+    - /usr/bin/salt * test.ping
     - /usr/bin/salt-call state*
     - /usr/bin/salt-call service*
     - /usr/bin/salt-call pillar*