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_marks.vim b/src/testdir/test_marks.vim
index 40242c0..dcf4904 100644
--- a/src/testdir/test_marks.vim
+++ b/src/testdir/test_marks.vim
@@ -1,3 +1,4 @@
+" Test for marks
 
 " Test that a deleted mark is restored after delete-undo-redo-undo.
 func Test_Restore_DelMark()