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_ex_z.vim

@@ -83,3 +83,5 @@ func Test_z_negative_lnum()
   call assert_equal(1, line('.'))
   bwipe!
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab