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_matchadd_conceal_utf8.vim b/src/testdir/test_matchadd_conceal_utf8.vim
index 2cdb062..83849e1 100644
--- a/src/testdir/test_matchadd_conceal_utf8.vim
+++ b/src/testdir/test_matchadd_conceal_utf8.vim
@@ -41,3 +41,5 @@
 
   quit!
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab