The new oldtest directory is in test/old/testdir. The reason for this is
that many tests have hardcoded the parent directory name to be
'testdir'.

Problem:    Fix for recovery and diff mode not tested.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes vim/vim#8352)

https://github.com/vim/vim/commit/3044324e8dccd470bd854cf7d9457232cc9c220e

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

Problem:    Using exclamation marks on :function.
Solution:   Use :func and :endfunc as usual.
https://github.com/vim/vim/commit/97f0eb169bf805c372b13c6bc9a03da2e75e3354

Add a missing change from patch 8.1.1875.

Problem:    Restarting Insert mode in prompt buffer too often when a callback
            switches windows and comes back. (Sean Dewar)
Solution:   Do not set "restart_edit" when already in Insert mode.
https://github.com/vim/vim/commit/34c20ff85b87be587ea5d0398812441b502ee6a5

As Test_prompt_switch_windows is skipped, implement it in prompt_buffer_spec.

Replace the 50ms term_wait calls with poke_eventloop (test seems to work anyway
without them, so maybe they're not required?)

The new test does include a duplicate screen test that may generate a "screen
test succeeded immediately" warning, but this is done to match the Vim test.

Problem:    Using CTRL-O in a prompt buffer moves cursor to start of the line.
Solution:   Do not move the cursor when restarting edit. (closes vim/vim#7330)
https://github.com/vim/vim/commit/ee8b787bcd15f63a938243770065e704c9b5c85f

Test_prompt_editing is skipped, so edit the Lua test in prompt_buffer_spec.

Problem:    Cannot backspace in prompt buffer after using cursor-left. (Maxim
            Kim)
Solution:   Ignore "arrow_used" in a prompt buffer. (closes vim/vim#7281)
https://github.com/vim/vim/commit/6f6244855fbce5aaa718cd5001a29aac3c5c15d6

cmdchar_todo wasn't adapted properly for Nvim's state system, which caused it to
be a dead store and such was removed in #11900.

Re-introduce cmdchar_todo properly.

Problem:    Writing to prompt buffer interferes with insert mode.
Solution:   Use win_enter() instead of just setting "curwin". (Ben Jackson,
            closes vim/vim#7035)
https://github.com/vim/vim/commit/4537bcc88956f86267c25edf8008e0dbde598652

Vim test will be skipped, so add a Lua test.
The problem boils down to the use of aucmd_restbuf in a callback, so just test
that (via nvim_buf_set_lines).

Problem:    Not easy to switch between prompt buffer and other windows.
Solution:   Accept CTRL-W commands in Insert mode.  Start and stop Insert mode
            as one would expect.
https://github.com/vim/vim/commit/6d41c78e353b630bc1a72cbff9160311d2a81e8c

Cherry-pick channel.txt change from:
https://github.com/vim/vim/commit/d2f3a8b8787333abf2300d38836b196955f10c00

b_prompt_insert was already ported.

These functions were ported with the vim-patch token, but didn't actually port
the method call support that was in their patches (method call syntax wasn't
ported yet).

Add the missing method call support and latest docs for:
 - assert_nobeep:
   https://github.com/vim/vim/commit/5b8cabfef7c3707f3e53e13844d90e5a217e1e84
 - buffer_name, buffer_number: (obsolete)
   https://github.com/vim/vim/commit/a8eee21e75324d199acb1663cb5009e03014a13a
 - charidx:
   https://github.com/vim/vim/commit/17793ef23aae0bc94539390ccfe5e63b0ad39ff2
 - flatten:
   https://github.com/vim/vim/commit/077a1e670ad69ef4cefc22103ca6635bd269e764
 - prompt_getprompt:
   https://github.com/vim/vim/commit/077cc7aa0e0c431e97795612374fe17fe7c88803
 - searchcount:
   https://github.com/vim/vim/commit/e8f5ec0d30b629d7166f0ad03434065d8bc822df
 - strptime:
   https://github.com/vim/vim/commit/10455d43fef041309ce0613fa792c635dd71e3a8
 - win_gettype:
   https://github.com/vim/vim/commit/00f3b4e007af07870168bf044cecc9d544483953
 - win_splitmove:
   https://github.com/vim/vim/commit/d20dcb3d011da6111153109f6e46fbd5c7fe9fb6

Also fix assert_beeps, assert_nobeep and getenv to accept exactly one argument.
Previously, they could erroneously accept one or more.