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_xxd.vim b/src/testdir/test_xxd.vim
index 37c0c57..4eef7a4 100644
--- a/src/testdir/test_xxd.vim
+++ b/src/testdir/test_xxd.vim
@@ -1,4 +1,5 @@
 " Test for the xxd command
+
 if empty($XXD) && executable('..\xxd\xxd.exe')
   let s:xxd_cmd = '..\xxd\xxd.exe'
 elseif empty($XXD) || !executable($XXD)
@@ -228,3 +229,5 @@
   call assert_match("xxd V1.10 .* by Juergen Weigert", join(getline(1, 3)))
   bwipe!
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab