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:  plugin/shada.lua does not respect the `+` flag of 'cpoptions'
          when writing a *.shada buffer into a different file.

Solution: Check the presence of `+` in 'cpoptions' and whether the
          buffer is written to a different file in BufWriteCmd for
          *.shada before resetting the 'modified' option of the buffer.

This little defect appeared after plugin/shada.vim was rewritten into
Lua in #34725.

Problem

Similar to https://github.com/neovim/neovim/pull/33257, but for the date
component. When timezone is behind UTC the epoch date of individual
components mismatch causing the test to fail.

Solution

The `epoch` variable is created using `os.date` with a `!`, which means
format the date in UTC instead of local timezone:
https://www.lua.org/manual/5.1/manual.html#pdf-os.date.

Since the individual components like year, month, and day are expected
to match `epoch` they should all be created using a `!` as well.

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.

Problem: When running functional tests locally, test `syntax/shada.vim works`
fails if the local timezone is not a whole number of hours ahead of UTC.

Solution: Use '!%M' for minute format so that UTC is used in the expected
timestamp instead of the local timezone, just like '%H' for hours.

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.

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.