Problem: Traditionally, the ruler in the last line is one cell shorter
than in the statusline, leaving the last cell of the screen blank.
According to code comments, this is in order to prevent unwanted
scrolling on "some" (unspecified, but presumably ancient) terminals.
Berkeley vi is more specific in its `vs_modeline` function: dumb
terminals with hardware scroll, SunOS 4.1.1 and Ultrix 4.2 curses.
(n)curses still has a similar limitation in `(w)addstr`, but apparently
only for historical reasons.

Maintaining the different widths leads to awkward inconsistencies when
the ruler is configured with 'rulerformat', except for the special case
where it contains a top-level `%=`. Shifting the ruler in the last line
to the left would be a solution, but the empty cell at the end doesn't
seem to be relevant anymore.

Solution: extend the ruler in the last line all the way to the right
edge of the screen, just like in the statusline. The exact same amount
of place will be available to the rest of the UI as before.

BREAKING CHANGE:
- the default ruler width is now 18 cells
- the last cell of the screen is no longer empty

Closes #41076

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:  Hidden windows are redrawn unnecessarily.
Solution: If a window is hidden, don't bother drawing it.

validate scrolling behavior in the presence of floating windows.

Problem:
When the terminal shrinks, Neovim tries to recompute the layout and
adjust every window so that the total width matches the new Columns. But
in the scenario of numerous windows split horizontally, there is
not enough space for all of them. The frame width (topframe->fr_width) never
reaches the new Columns value, the logic tries to redistribute this negative
space across child frames, but it triggers the assert.

Solution:
Skip this case in `win_update`.

Problem:
- `n.spawn()` is misleading because it also connects RPC, it's not just
  "spawning" a process.
- It's confusing that `n.spawn()` and `n.spawn_argv()` are separate.

Solution:
- Replace `n.spawn()`/`n.spawn_argv()` with a single function `n.new_session()`.
  This name aligns with the existing functions `n.set_session`/`n.get_session`.
    - Note: removes direct handling of `prepend_argv`, but I doubt that was
      important or intentional. If callers want to control use of `prepend_argv`
      then we should add a new flag to `test.session.Opts`.
- Move `keep` to first parameter of `n.new_session()`.
- Add a `merge` flag to `test.session.Opts`
- Mark `_new_argv()` as private. Test should use clear/new_session/spawn_wait
  instead.

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.