patch 8.0.0525: completion for user command argument not tested

Solution:   Completion for user command argument not tested.
Problem:    Add a test.
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 77c9170..1b0597e 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -348,6 +348,15 @@
   bw!
 endfunc
 
+func Test_cmdline_complete_user_cmd()
+  command! -complete=color -nargs=1 Foo :
+  call feedkeys(":Foo \<Tab>\<Home>\"\<cr>", 'tx')
+  call assert_equal('"Foo blue', @:)
+  call feedkeys(":Foo b\<Tab>\<Home>\"\<cr>", 'tx')
+  call assert_equal('"Foo blue', @:)
+  delcommand Foo
+endfunc
+
 " using a leading backslash here
 set cpo+=C