patch 8.1.0559: command line completion not sufficiently tested

Problem:    Command line completion not sufficiently tested.
Solution:   Add more tests. (Dominique Pelle, closes #3622)
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index b896c3d..cb1117e 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -396,6 +396,18 @@
   %argd
 endfunc
 
+func Test_argdelete_completion()
+  args foo bar
+
+  call feedkeys(":argdelete \<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"argdelete bar foo', @:)
+
+  call feedkeys(":argdelete x \<C-A>\<C-B>\"\<CR>", 'tx')
+  call assert_equal('"argdelete x bar foo', @:)
+
+  %argd
+endfunc
+
 " Tests for the :next, :prev, :first, :last, :rewind commands
 func Test_argpos()
   call Reset_arglist()