[vim/vim] tabpage: closing a tab page loses the alternate tab page (PR #20973)

```
Problem:  Closing the current tab page resets the alternate tab page, even
          when that is another tab page which still exists, so that
          CTRL-Tab stops working (igorlfs).
Solution: Restore the last used tab page after entering another one to
          close the current one.
```

related: #20965

---

Closing the current tab page clears the alternate tab page even when the
alternate is a different tab page that still exists:

    :tabnew
    :tabnew
    :tabfirst
    :tablast
    :echo tabpagenr('#')    " 1
    :tabclose
    :echo tabpagenr('#')    " 0, while tab page 1 is still there

`close_last_window_tabpage()` enters another tab page before closing the
current one, which makes the tab page about to be closed the last used one,
and `free_tabpage()` then resets it.

This does not fix the steps in #20965. There the alternate tab page becomes
the current one after the close, so there is nothing left to go back to
without keeping a history of visited tab pages. That would be out of line
with the rest of Vim, where the alternate file (`#`) and the previous window
(`CTRL-W p`) are single entries as well, and it would make the destination of
CTRL-Tab hard to predict.
You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/20973

-- Commit Summary --

  * tabpage: closing a tab page loses the alternate tab page

-- File Changes --

    M src/testdir/test_tabpage.vim (17)
    M src/window.c (6)

-- Patch Links --

https://github.com/vim/vim/pull/20973.patch
https://github.com/vim/vim/pull/20973.diff

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