patch 8.2.1432: various inconsistencies in test files

Problem:    Various inconsistencies in test files.
Solution:   Add modelines where they were missing.  Use Check commands instead
            of silently skipping over tests.  Adjust indents and comments.
            (Ken Takata, closes #6695)
diff --git a/src/testdir/test_fold.vim b/src/testdir/test_fold.vim
index 5a4632a..615fb44 100644
--- a/src/testdir/test_fold.vim
+++ b/src/testdir/test_fold.vim
@@ -557,9 +557,7 @@
 
 " test syntax folding
 func Test_fold_syntax()
-  if !has('syntax')
-    return
-  endif
+  CheckFeature syntax
 
   enew!
   set fdm=syntax fdl=0