patch 8.2.0693: closure using argument not tested

Problem:    Closure using argument not tested.
Solution:   Add a test, make it work.
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index fe47513..c06cc23 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -700,5 +700,26 @@
   unlet g:Read
 enddef
 
+def MakeArgRefs(theArg: string)
+  let local = 'loc_val'
+  g:UseArg = {s -> theArg .. '/' .. local .. '/' .. s}
+enddef
+
+def MakeArgRefsVarargs(theArg: string, ...rest: list<string>)
+  let local = 'the_loc'
+  g:UseVararg = {s -> theArg .. '/' .. local .. '/' .. s .. '/' .. join(rest)}
+enddef
+
+def Test_closure_using_argument()
+  MakeArgRefs('arg_val')
+  assert_equal('arg_val/loc_val/call_val', g:UseArg('call_val'))
+
+  MakeArgRefsVarargs('arg_val', 'one', 'two')
+  assert_equal('arg_val/the_loc/call_val/one two', g:UseVararg('call_val'))
+
+  unlet g:UseArg
+  unlet g:UseVararg
+enddef
+
 
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker