patch 8.0.1689: no tests for xxd

Problem:    No tests for xxd.
Solution:   Add a test. (Christian Brabandt)

src/testdir/runtest.vim

@@ -199,7 +199,11 @@ func FinishTesting()
     write
   endif
 
-  let message = 'Executed ' . s:done . (s:done > 1 ? ' tests' : ' test')
+  if s:done == 0
+    let message = 'NO tests executed'
+  else
+    let message = 'Executed ' . s:done . (s:done > 1 ? ' tests' : ' test')
+  endif
   echo message
   call add(s:messages, message)
   if s:fail > 0