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)
Problem: FreeBSD: test for sourcing utf-8 is skipped.
Solution: Run the matchadd_conceal test separately to avoid that setting
'term' to "ansi" causes problems for other tests. (Ozaki Kiichi,
closes#5721)
