vim-patch:8.2.4492: no error if an option is given a value with ":let &opt = val" (#19670)

Problem:    No error if an option is given an invalid value with
            ":let &opt = val".
Solution:   Give the error. (closes vim/vim#9864)
8ccbbeb620

src/nvim/testdir/test_normal.vim

@@ -1975,7 +1975,8 @@ func Test_normal31_r_cmd()
   " using CTRL-Y and CTRL-E.
   " Different code paths are used for utf-8 and latin1 encodings
   set showmatch
-  for enc in ['latin1', 'utf-8']
+  " for enc in ['latin1', 'utf-8']
+  for enc in ['utf-8']
     enew!
     let &encoding = enc
     call setline(1, [' {a}', 'xxxxxxxxxx', '      [b]'])