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.

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

This is the first installment of a multi-PR series significantly
refactoring how highlights are being specified.

The end goal is to have a base set of 20 ish most common highlights,
and then specific files only need to add more groups to that as needed.

As a complicating factor, we also want to migrate to the new default
color scheme eventually. But by sharing a base set, that future PR
will hopefully be a lot smaller since a lot of tests will be migrated
just simply by updating the base set in place.

As a first step, fix the anti-pattern than Screen defaults to ignoring
highlights. Highlights are integral part of the screen state, not
something "extra" which we only test "sometimes". For now, we still
allow opt-out via the intentionally ugly

  screen._default_attr_ids = nil

The end goal is to get rid of all of these eventually (which will be
easier as part of the color scheme migration)

- remove helpers.cur*meths
- remove helpers.nvim

This is the command invoked repeatedly to make the changes:

    :%s/^\(.*\)|\%(\*\(\d\+\)\)\?$\n\1|\%(\*\(\d\+\)\)\?$/\=submatch(1)..'|*'..(max([str2nr(submatch(2)),1])+max([str2nr(submatch(3)),1]))/g

Problem:
Some tests fail with $SHELL=fish #6172
Related: https://github.com/neovim/neovim/pull/6176

Solution:
Replace "echo -n" with "printf", because "echo" in sh may be provided
as a shell builtin, which does not accept an "-n" flag to avoid a
trailing newline (e.g. on macos). "printf" is more portable (defined by
POSIX) and it does not output a trailing newline by itself.
Fixes #6172

TODO:
Other test failures may be related to "session leader" issue: https://github.com/neovim/neovim/issues/2354
Checked by running `:terminal ./build/bin/tty-test` from Nvim with
`shell=/bin/fish` (inherited from `$SHELL`) and it indeed complains
about "process does not own the terminal". With `shell=sh` it doesn't complain. And
unsetting `$SHELL` seems to make `nvim` to fall back to `shell=sh`.

    FAILED   test/functional/terminal/tui_spec.lua @ 1017: TUI paste: terminal mode
    test/functional/terminal/tui_spec.lua:1024: Row 1 did not match.
    Expected:
      |*tty ready                                         |
      |*{1: }                                                 |
      |*                                                  |
      |                                                  |
      |{5:^^^^^^^                                           }|
      |{3:-- TERMINAL --}                                    |
      |{3:-- TERMINAL --}                                    |
    Actual:
      |*process does not own the terminal                 |
      |*                                                  |
      |*[Process exited 2]{1: }                               |
      |                                                  |
      |{5:^^^^^^^                                           }|
      |{3:-- TERMINAL --}                                    |
      |{3:-- TERMINAL --}                                    |

    To print the expect() call that would assert the current screen state, use
    screen:snapshot_util(). In case of non-deterministic failures, use
    screen:redraw_debug() to show all intermediate screen states.

    stack traceback:
        test/functional/ui/screen.lua:622: in function '_wait'
        test/functional/ui/screen.lua:352: in function 'expect'
        test/functional/terminal/tui_spec.lua:1024: in function <test/functional/terminal/tui_spec.lua:1017>

    FAILED   test/functional/terminal/tui_spec.lua @ 1551: TUI forwards :term palette colors with termguicolors
    test/functional/terminal/tui_spec.lua:1567: Row 1 did not match.
    Expected:
      |*{1:t}ty ready                                         |
      |                                                  |
      |*                                                  |
      |                                                  |
      |{2:^^^^^^^                                           }|
      |                                                  |
      |{3:-- TERMINAL --}                                    |
    Actual:
      |*{1:p}rocess does not own the terminal                 |
      |                                                  |
      |*[Process exited 2]                                |
      |                                                  |
      |{2:^^^^^^^                                           }|
      |                                                  |
      |{3:-- TERMINAL --}                                    |

    To print the expect() call that would assert the current screen state, use
    screen:snapshot_util(). In case of non-deterministic failures, use
    screen:redraw_debug() to show all intermediate screen states.

    stack traceback:
        test/functional/ui/screen.lua:622: in function '_wait'
        test/functional/ui/screen.lua:352: in function 'expect'
        test/functional/terminal/tui_spec.lua:1567: in function <test/functional/terminal/tui_spec.lua:1551>