Fix has_spec.lua for v9.1.0000

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`.

Builds with -DNDEBUG do not contain the compilation string, so the test
will fail even if nvim is built with unibilium.

Similarly, zig builds only report "Compilation: TODO", so they also fail
the test even when build with unibilium

Detect both scenarios and skip the test, rather than failing it.

Problem: has("terminfo") test fails on local runs because it expects
"HAVE_UNIBILIUM " in the :version info to mean that nvim was built
without unibilium.

Solution: Assume nvim is built with unibilium if HAVE_UNIBILIUM is
present in the version string and is NOT followed by 0, false, or off.

This isn't affecting CI runs because when the test detects that it is
running in CI it doesn't use the :version string at all.

Resolves https://github.com/neovim/neovim/issues/37456

Group up to 15 vimpatch numbers in 1 line to guard against
'make formatc'.
1-liner for vim_versions, num_patches.

Automate '*Version' to remove version.h macros.

'-V1 -v' lists merged Vim versions.

Problem:
No way to detect at runtime if the build includes unibilium (or whatever
terminfo layer we swap it with later).

Solution:
Support `has('terminfo')`.

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.

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.