vim-patch:9.1.0803: tests: no error check when setting global 'isk'

Problem:  tests: no error check when setting global 'isk'
Solution: also parse and check global 'isk' value (Milly)

closes: vim/vim#15915

5e7a6a4a10

Co-authored-by: Milly <milly.ca@gmail.com>

src/nvim/options.lua

@@ -4459,7 +4459,7 @@ return {
     {
       abbreviation = 'isk',
       alloced = true,
-      cb = 'did_set_isopt',
+      cb = 'did_set_iskeyword',
       defaults = { if_true = '@,48-57,_,192-255' },
       deny_duplicates = true,
       desc = [=[