patch 8.2.2659: eval test fails because for loop on string works

Problem:    Eval test fails because for loop on string works.
Solution:   Check looping over function reference fails.
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index 95f5b24..4549f48 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -66,7 +66,7 @@
 
 func Test_for_invalid()
   call assert_fails("for x in 99", 'E714:')
-  call assert_fails("for x in 'asdf'", 'E714:')
+  call assert_fails("for x in function('winnr')", 'E714:')
   call assert_fails("for x in {'a': 9}", 'E714:')
 
   if 0