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_paste.vim b/src/testdir/test_paste.vim
index 194ae7d..8995b97 100644
--- a/src/testdir/test_paste.vim
+++ b/src/testdir/test_paste.vim
@@ -135,9 +135,7 @@
 endfunc
 
 func Test_xrestore()
-  if !has('xterm_clipboard')
-    return
-  endif
+  CheckFeature xterm_clipboard
   let display = $DISPLAY
   new
   call CheckCopyPaste()