Add Salt 2018.3 tests and update test pillar

Change-Id: I91c8894200ff97a6ef8f5e1c0e5829f84e3e2412
diff --git a/tests/pillar/client_single.sls b/tests/pillar/client_single.sls
index 87507ec..dbd4d8a 100644
--- a/tests/pillar/client_single.sls
+++ b/tests/pillar/client_single.sls
@@ -9,6 +9,11 @@
 linux:
   system:
     enabled: true
+    group:
+      jdoe:
+        enabled: true
+        name: jdoe
+        system: true
     user:
       jdoe:
         enabled: true