patch 8.1.1279: cannot set 'spellang' to "sr@latin"

Problem:    Cannot set 'spellang' to "sr@latin". (Bojan Stipic)
Solution:   Allow using '@' in 'spellang'. (closes #4342)
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index bd3f80c..74c725f 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -125,6 +125,7 @@
       \ 'sessionoptions': [['', 'blank', 'help,options,slash'], ['xxx']],
       \ 'signcolumn': [['', 'auto', 'no'], ['xxx', 'no,yes']],
       \ 'spellfile': [['', 'file.en.add'], ['xxx', '/tmp/file']],
+      \ 'spelllang': [['', 'xxx', 'sr@latin'], ['not&lang', "that\\\rthere"]],
       \ 'spellsuggest': [['', 'best', 'double,33'], ['xxx']],
       \ 'switchbuf': [['', 'useopen', 'split,newtab'], ['xxx']],
       \ 'tagcase': [['smart', 'match'], ['', 'xxx', 'smart,match']],