patch 9.1.0783: 'spell' option setting has problems

Problem:  'spell' option setting has problems
Solution: correctly check for comma for 'spellfile' option,
          remove unnecessary checks, refactor slightly (Milly)

closes: #15873

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_spellfile.vim b/src/testdir/test_spellfile.vim
index 8abb680..dd12698 100644
--- a/src/testdir/test_spellfile.vim
+++ b/src/testdir/test_spellfile.vim
@@ -1154,7 +1154,7 @@
 " 'spellfile' accepts '@' on top of 'isfname'.
 def Test_spellfile_allow_at_character()
   mkdir('Xtest/the foo@bar,dir', 'p')
-  &spellfile = './Xtest/the foo@bar,dir/Xspellfile.add'
+  &spellfile = './Xtest/the foo@bar\,dir/Xspellfile.add'
   &spellfile = ''
   delete('Xtest', 'rf')
 enddef