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_preview.vim b/src/testdir/test_preview.vim
index 1bf1d3f..b7b908e 100644
--- a/src/testdir/test_preview.vim
+++ b/src/testdir/test_preview.vim
@@ -1,4 +1,5 @@
 " Tests for the preview window
+
 source check.vim
 CheckFeature quickfix