Problem: Argument type mismatches are excluded from EmmyLua checks.

Solution: Correct runtime annotations and calls, document local checker
workarounds, and enable the diagnostic.

AI-assisted

Problem:
When 'shellslash' is set, exepath() returns forward-slashed paths.
Passing that path to vim.system() can prevent cmd.exe from launching.

Solution:
Convert the resolved executable path to native separators before spawning.
Add a Windows regression test for cmd.exe with 'shellslash' enabled.

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`.

    FAILED   …/plugin/pack_spec.lua:92: …/plugin/pack_spec.lua @ 70: vim.pack update() can use lockfile revision as a target
    …/plugin/pack_spec.lua:92: …/plugin/pack_spec.lua:70: process still exists
    stack traceback:
    D:/a/neovim/neovim/test/functional/testnvim/exec_lua.lua:124: in function 'system_sync'
    …/plugin/pack_spec.lua:92: in function 'git_cmd'
    …/plugin/pack_spec.lua:113: in function 'git_add_commit'
    …/plugin/pack_spec.lua:1209: in function <…/plugin/pack_spec.lua:1173>

fix #40167

    FAILED   2 tests, listed below:
    FAILED   …/lua/system_spec.lua:124: …/lua/system_spec.lua @ 44: vim.system (async) supports timeout
    …/lua/system_spec.lua:124: …/lua/system_spec.lua:44: process still exists
    stack traceback:
    D:/a/neovim/neovim/test/functional/testnvim/exec_lua.lua:124: in function 'system'
    …/lua/system_spec.lua:124: in function <…/lua/system_spec.lua:118>
    FAILED   …/lua/system_spec.lua:111: …/lua/system_spec.lua @ 22: vim.system (sync) can set environment with clear_env = true and env = nil
    …/lua/system_spec.lua:111: …/lua/system_spec.lua:22: process still exists
    stack traceback:
    D:/a/neovim/neovim/test/functional/testnvim/exec_lua.lua:124: in function 'system'
    …/lua/system_spec.lua:111: in function <…/lua/system_spec.lua:97>

Problem:
Listing individual "skipped" notices for each non-actionable or
NVIM_TEST_INTEG test is noisy and makes the "skipped" list overwhelming
and less meaningful.

Solution:
- Mark non-actionable tests with "N/A".
- Report NVIM_TEST_INTEG tests as a separate 1-line summary.

This is essentially the #35991 fix for the #35984 issue,
but applied to vim.system().