Problem:
msgpack#strptime() finds a timestamp by bisecting strftime() output, and
brackets the search with the extreme UTC offsets. The lower bound
subtracts 12 hours, but -12:00 is the westernmost offset, which produces
the *largest* timestamp for a given local time. The easternmost offset
is +14:00, so in any zone east of UTC+12 the search can start above the
target and the function throws:

    internal-start-string:Internal error: start > string

With TZ=GMT-14, four of the five timestamps in msgpack_spec.lua fail,
as do three tests in shada_spec.lua, which reaches the same function
through shada#strings_to_sd().

Solution:
Subtract 14 hours instead. The upper bound is already generous enough at
+14, and widening a bisection bracket downwards cannot change the result
in zones that worked before.

The test disables the python3 provider, because msgpack#strptime() only
uses the Vimscript implementation changed here when no provider answers,
and otherwise hands the work to datetime.strptime().

AI-assisted

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`.

Use only a single clear() call in some test/functional/vimscript/ test
files whose test cases have very little side effect.

A downside of using a single clear() is that if a crash happens in one
test case, all following test cases in the same file will also fail, but
these functionalities and tests don't change very often.

This is a breaking change which will make refactor of typval and shada
code a lot easier. In particular, code that would use or check for
v:msgpack_types.binary in the wild would be broken. This appears to be
rarely used in existing plugins.

Also some cases where v:msgpack_type.string would be used to represent a
binary string of "string" type, we use a BLOB instead, which is
vimscripts native type for binary blobs, and already was used for BIN
formats when necessary.

msgpackdump(msgpackparse(data)) no longer preserves the distinction
of BIN and STR strings. This is very common behavior for
language-specific msgpack bindings. Nvim uses msgpack as a tool to
serialize its data. Nvim is not a tool to bit-perfectly manipulate
arbitrary msgpack data out in the wild.

The changed tests should indicate how behavior changes in various edge
cases.

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.

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