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`.

Same idea as a7be4b7bf857, but that only showed the context if the
length of the string differed. Since these tests check both string
length and string content, the ctx should be provided for both.

    ERROR    test/unit/testutil.lua @ 797: vim_snprintf() positional arguments
    test/unit/testutil.lua:769: test/unit/testutil.lua:753: (string) '
    test/unit/strings_spec.lua:159: snprintf(buf, 4, "%1$0.*2$b", 12ULL, cdata<int>: 0xf78c8ed8) = 001100
    Expected objects to be the same.
    Passed in:
    (string) '000'
    Expected:
    (string) '001''

When 9.0.1856 was ported, the numbers being formatted as binary were cast
to "unsigned int" rather than uvarnumber_T, as is done upstream.

Since these assertions all use a common function to perform the test
assertions, it's difficult to figure out which test failed:

    ERROR    test/unit/testutil.lua @ 785: vim_snprintf() positional arguments
    test/unit/testutil.lua:757: test/unit/testutil.lua:741: (string) '
    test/unit/strings_spec.lua:143: Expected objects to be the same.
    Passed in:
    (number) 6400
    Expected:
    (number) 6'
    exit code: 256

Adding context to the assertion makes it clearer what the problem is:

    ERROR    test/unit/testutil.lua @ 785: vim_snprintf() positional arguments
    test/unit/testutil.lua:757: test/unit/testutil.lua:741: (string) '
    test/unit/strings_spec.lua:149: snprintf(buf, 0, "%1$0.*2$b", cdata<unsigned int>: 0xf78d0f38, cdata<int>: 0xf78dc4e0) = 001100
    Expected objects to be the same.
    Passed in:
    (number) 6400
    Expected:
    (number) 6'
    exit code: 256

LuaJIT does not handle -0.0 correctly in 'dual number mode' (which is
the default, and only supported mode for LuaJIT arm64). If LuaJIT is
forced to use 'dual number mode' on X64 (where the default is single),
this test will fail in the same manner.

Fix this by using tonumber("-0.0") instead of a -0.0 literal.

See: https://github.com/LuaJIT/LuaJIT/issues/858

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

Problem:
Not all Lua code is checked by stylua. Automating code-style is an
important mechanism for reducing time spent on accidental
(non-essential) complexity.

Solution:
- Enable lintlua for `test/unit/` directory.
- TODO: only `test/functional/` remains unchecked.

previous: 45fe4d11add933df76a2ea4bf52ce8904f4a778b
previous: 517f0cc634b985057da5b95cf4ad659ee456a77e

Problem:  issues with formatting positional arguments
Solution: fix them, add tests and documentation

closes: vim/vim#12140
closes: vim/vim#12985

Tentatively fix message_test. Check NULL ptr.

https://github.com/vim/vim/commit/aa90d4f031f73a34aaef5746931ea746849a2231

Co-authored-by: Christ van Willegen <cvwillegen@gmail.com>

Problem: duplicate test in message_test.c
Solution: Remove duplicate test and make functions static

closes: vim/vim#12803

https://github.com/vim/vim/commit/7772c93a3ec1ce60469e9aea9899687ae1141f34