Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

Problem:
Continue separators normalization, and try to keep it at the
nvim <-> external boundary, e.g., `fn.xxx`, `api.xxx`, `:xxx`

Solution:
some key changes
- normalize `$HOME-windows`
- normalize entry points of changing directory
  - `nvim_set_current_dir()`, `chdir()`
- normalize the named pipe
  - `--listen` arg, `--server` arg, `:restart`
  - `serverstart()`, `sockconnect()`, `serverstop()`
- make `expand()` respect 'shellslash' again
- clean up `did_set_shellslash`
- replace `forward_slash` with `TO_SLASH`
- remove obsolete `TMP_PATHSEPSTR`
- remove `slash_adjust` in `do_autocmd_dirchanged`?
- make `fnamemodify()` always return `/` (except when substituting
  separators via `:s`, `:gs` ?)

Note:
- these funcs still apply `slash_adjust` on return, as before:
  `:pwd`, `chdir()`, `exepath()`, `getcwd()`
- these funcs alwarys return `/`, unlike before the normalization PRs
  - `getcompletion()`, `finddir()`, `findfile()`

Also
- clean up code, comments, formatting and tests
- move `TO_SLASH` from `f_chdir` to `changedir_func`
- move `TO_SLASH` from `f_bufadd` to `buflist_new`

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

Problem:
On Windows, path separators may become inconsistent for various reasons,
which makes normalization quite painful.

Solution:
Normalize paths to `/` at the entry boundaries and always use it
internally, converting back only in rare cases where `\` is really
needed (e.g. cmd.exe/bat scripts?).

This is the first commit in a series of incremental steps.

Note:
* some funcs won't respect shellslash. e.g. `expand/fnamemodify`
* some funcs still respect shellslash, but will be updated in a follow
  PR. e.g. `ex_pwd/f_chdir/f_getcwd`
* uv's built-in funcs always return `\`. e.g. `uv.cwd/uv.exepath`

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

This is a better way to prevent parallel tests from interfering with
each other, as there are many ways files can be created and deleted in
tests, so enforcing different file names is hard.

Using $TMPDIR can also work in most cases, but 'backipskip' etc. have
special defaults for $TMPDIR.

Symlink runtime/, src/, test/ and README.md to Xtest_xdg dir to make
tests more convenient (and symlinking test/ is required for busted).

Also, use README.md instead of test/README.md in the Ex mode inccommand
test, as test/README.md no longer contains 'N' char.

Define a CMake target for every subdirectory of test/functional that
contains functional tests, and a functionaltest_parallel target that
depends on all those targets, allowing multiple test runners to run in
parallel.

On CI, use at most 2 parallel test runners, as using more may increase
system load and make tests unstable.

Problem:  Windows: Vim always adds the current directory to search path.
          This should only happen when using cmd.exe as 'shell'. For
          example, powershell won't run binaries from the current
          directory.
Solution: Only add current directory to system path, when using cmd.exe
          as 'shell'.

related: vim/vim#10341
related: 083ec6d9a3b7

https://github.com/vim/vim/commit/4d87c9742a544a58c05cb34d4fbaac47e410b369

Co-authored-by: Christian Brabandt <cb@256bit.org>

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.