It is perfectly fine and expected to detach from the screen just by
the UI disconnecting from nvim or exiting nvim. Just keep detach() in
screen_basic_spec, to get some coverage of the detach method itself.

This avoids hang on failure in many situations (though one could argue
that detach() should be "fast", or at least "as fast as resize",
which works in press-return already).

Never use detach() just to change the size of the screen, try_resize()
method exists for that specifically.

"attr_ignore" is an anti-pattern, with snapshot_util()
just include all the highlights already.

The call to plugin_helpers.reset() is redundant with the clear() call
above it.  Probably just a copy-paste mistake.

Avoids exit_event race #8813.

Helped-by: Björn Linse <bjorn.linse@gmail.com>

closes #10267

Lua (not LuaJIT) complains about the "^[[" strings inside the expect,
since it sees them as nested quotes.  Change the quoting to [=[ ]=] to
avoid the issue.