patch 9.0.1218: completion includes functions that don't work

Problem:    Completion includes functions that don't work.
Solution:   Skip functions that are not implemented. (Kota Kato,
            closes #11845)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index a832306..9eca918 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -489,6 +489,12 @@
   let l = getcompletion('paint', 'function')
   call assert_equal([], l)
 
+  if !has('ruby')
+    " global_functions[] has an entry but it doesn't have an implemention
+    let l = getcompletion('ruby', 'function')
+    call assert_equal([], l)
+  endif
+
   let Flambda = {-> 'hello'}
   let l = getcompletion('', 'function')
   let l = filter(l, {i, v -> v =~ 'lambda'})