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'.

'shellxquote' Nvim default was adjusted in: 131aad953c007d382cbff1d2560471b29975da87
The use of "/s" is different than Vim, and may avoid the need for `shellxquote="&|<>()@^`.

For the other shells, Nvim intentionally does not fiddle with the
various "shell*" options if 'shell' is set by the user: if the user sets
'shell', they are expected to set other "shell*" options correctly.

(Most of this patch is intentionally dropped, it adds far too much
special-cases for almost zero purpose: ":help shell-powershell" makes it
easy to choose powershell without spreading special-cases throughout the
codebase, randomly changing slash behavior, etc.)

Problem:    Shell options are not set properly for PowerShell.
Solution:   Use better option defaults. (Mike Willams, closes vim/vim#8459)

https://github.com/vim/vim/commit/127950241e84c822d3c50f46a00d42a70d2d5cb6

Co-authored-by: Mike Williams <mikew@globalgraphics.com>

Problem:    Not enough testing for shell use.
Solution:   Add a bit more testing. (Yegappan Lakshmanan, closes vim/vim#8469)

https://github.com/vim/vim/commit/ffec6dd16a766180429addaa78928c773a3c9832

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

Problem:    Testing the shell option is incomplete and spread out.
Solution:   Move shell tests to one file and increase coverage. (Yegappan
            Lakshmanan, closes vim/vim#8464)

https://github.com/vim/vim/commit/054794c20f6322bbd9482c4124041dc0a140c78e

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