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:
When `:!` writes shell output to a buffer, write_output() splits on `\r`, `\n`,
and `\r\n`, replacing the terminator byte with NUL. For a binary-mode buffer
this is wrong: `\r` should be preserved verbatim, not treated as a line
terminator. This wrong behavior causes a file like `\r\n` round-trips through
`:%!cat` to `\n`.

This was masked when 'shelltemp' was enabled, because output went through a temp
file and the regular file I/O path handled binary-mode correctly. Switching the
default to 'noshelltemp' exposed the bug, since output is now piped directly
into write_output().

Solution:
In `write_output()`, skip the `\r` and `\r\n` splits for a binary-mode buffer;
only split on `\n`.

Problem:
On Windows, `:!echo тест` shows `????` because the console code page defaults to a legacy ANSI encoding (e.g. CP1252) instead of `UTF-8`

Solution:
Call `SetConsoleOutputCP(CP_UTF8)` and `SetConsoleCP(CP_UTF8)` in `do_os_system()` before spawning child processes, and restore the original values after. It covers both `:!` and `system()` since they both go through `do_os_system()`

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.

On linux /dev/stdin is defined as a symlink to /proc/self/fd/0
This in turn is defined as a "magic" symlink which is allowed to point
to internal kernel objects which really does not have a file
name. As a glaring inconsistency, fopen("/proc/self/fd/0", "r")
works if fd was originally opened using pipe() but not using
socketpair(). As it happens UV_CREATE_PIPE does not create pipes
but creates socket pairs. These two unfortunate conditions
means that using /dev/stdin and similar does not work in
shell commands in nvim on linux. as a work around, override
libuv's descicion and create an actual pipe pair.

This change is not needed on BSD:s but done unconditionally for simplicity,
except for on windows where it is not done for stdout because of windows

fixes #35984

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

Also change job tests to use `nvim` instead of random programs like `ping`.

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.