vim-patch:8.2.4462: not enough testing for quickfix code

Problem:    Not enough testing for quickfix code.
Solution:   Add more tests. Fix uncovered problem. (Yegappan Lakshmanan,
            closes vim/vim#9839)
9c9be05b17

Omit Test_helpgrep_vim9_restore_cpo().
Cherry-pick test_quickfix.vim change from patch 8.2.0644.

src/nvim/window.c

@@ -4913,7 +4913,7 @@ win_T *buf_jump_open_win(buf_T *buf)
 }
 
 /// Jump to the first open window in any tab page that contains buffer "buf",
-/// if one exists.
+/// if one exists. First search in the windows present in the current tab page.
 /// @return the found window, or NULL.
 win_T *buf_jump_open_tab(buf_T *buf)
 {