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)

src/testdir/test_json.vim

@@ -323,3 +323,5 @@ func Test_json_encode_long()
   let json = json_encode([repeat('a', 3996)])
   call assert_equal(4000, len(json))
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab