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_increment_dbcs.vim

@@ -1,4 +1,5 @@
 " Tests for using Ctrl-A/Ctrl-X using DBCS.
+
 set encoding=cp932
 scriptencoding cp932