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)

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:    Tests failing because there is no error for float to string
            conversion.
Solution:   Change the check for failure to check for correct result.  Make
            some conversions strict in Vim9 script.

https://github.com/vim/vim/commit/3cfa5b16b06bcc034f6de77070fa779d698ab5e9

Co-authored-by: Bram Moolenaar <Bram@vim.org>