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)

src/testdir/test_termcodes.vim

@@ -270,7 +270,6 @@ endfunc
 func Test_term_mouse_middle_click_no_clipboard()
   if has('clipboard_working')
     throw 'Skipped: clipboard support works'
-    return
   endif
   new
   let save_mouse = &mouse