Before #31525 the prompts had a trailing space.
Also add a test for #7857.

Problem:  getchar_spec may fail when screen:expect_unchanged() doesn't
          wait long enough.
Solution: Add poke_eventloop() before screen:expect_unchanged().

Problem:  Cannot control cursor positioning of getchar().
Solution: Add "cursor" flag to {opts}, with possible values "hide",
          "keep" and "msg".

related: vim/vim#10603
closes: vim/vim#16569

https://github.com/vim/vim/commit/edf0f7db28f87611368e158210e58ed30f673098

Problem:  Vim9: Regression caused by patch v9.1.0646
Solution: Translate the function name before invoking it in call()
          (Yegappan Lakshmanan)

fixes: vim/vim#16430
closes: vim/vim#16445

https://github.com/vim/vim/commit/6289f9159102e0855bedc566636b5e7ca6ced72c

N/A patch:
vim-patch:8.2.4176: Vim9: cannot use imported function with call()

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

Problem:  Prompts are emitted as messages events, where cmdline events
          are more appropriate. The user input is also emitted as
          message events in fast context, so cannot be displayed with
          vim.ui_attach().
Solution: Prompt for user input through cmdline prompts.

Problem:  Lua callbacks for "msg_show" events with vim.ui_attach() are
          executed when it is not safe.
Solution: Disallow non-fast API calls for "msg_show" event callbacks.
          Automatically detach callback after excessive errors.
          Make sure fast APIs do not modify Nvim state.

Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.

In the api_info() output:

    :new|put =map(filter(api_info().functions, '!has_key(v:val,''deprecated_since'')'), 'v:val')
    ...

    {'return_type': 'ArrayOf(Integer, 2)', 'name': 'nvim_win_get_position', 'method': v:true, 'parameters': [['Window', 'window']], 'since': 1}

The `ArrayOf(Integer, 2)` return type didn't break clients when we added
it, which is evidence that clients don't use the `return_type` field,
thus renaming Dictionary => Dict in api_info() is not (in practice)
a breaking change.

- `alter_slashes` belongs in `testutil.lua`, not `testnvim.lua`.
- `alter_slashes` is an unusual name. Rename it to `fix_slashes`.
- invert its behavior, to emphasize that `/` slashes are the preferred,
  pervasive convention, not `\` slashes.