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:
* 'shellcmdflag' states that its default value is set according to the
value of 'shell', but this behavior is not yet implemented on Windows.
The same applies to 'shellpipe', 'shellredir', and 'shellxquote'.
* On Windows, Git is often installed in paths containing spaces, and we
still do not correctly resolve the sh executable name as described in
'shell'.
* On Windows, the default value of 'shellslash' is always `false`,
which causes Unix-like shells to interpret `\` in paths returned by
some functions as escape charaters.
Solution:
Use a simple rule table to detect common shells (e.g. `cmd`,
`powershell`, shells whose names contain `csh` or `sh`) and apply
best-effort defaults, while leaving more complex scenarios to user
configuration.
