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:
Malicious filename can execute code because of ":" cmdline expansion.

Solution:
Use `fnameescape()`.

fix https://github.com/neovim/neovim/issues/39914

Problem:
We should not use "\" (backslashes) except where absolutely required.
See references in https://github.com/neovim/neovim/pull/37729

Solution:
There is no reason to use "\" slashes in the trust db, so don't.

Problem:
`vim.secure.trust()` hashes an unchanged empty buffer as
a newline, so trusting an empty file by buffer never works.

Solution:
Hash unchanged empty-buffers `''` so buffer-based
trust matches the on-disk empty file.

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.

- Remove "state" directory after each test, so that a failure in one
  test won't interfere with later tests.
- Still make sure the trust file is empty at the end of each test.