tests/ui: make screen.lua use "linegrid" representation internally

PR #8221 took a short-cut when implementing the tests: screen.lua would
translate the linegrid highlight ids back into the old per-cell
attribute description.

Apart from cleaning up technical debt, this enables to check both rgb
and cterm colors in the same expect(), which previously was needlessly
restricted to ext_hlstate tests only.

test/functional/terminal/helpers.lua

@@ -52,7 +52,7 @@ local function screen_setup(extra_rows, command, cols, opts)
     [3] = {bold = true},
     [4] = {foreground = 12},
     [5] = {bold = true, reverse = true},
-    [6] = {background = 11},
+    -- 6 was a duplicate item
     [7] = {foreground = 130},
     [8] = {foreground = 15, background = 1}, -- error message
     [9] = {foreground = 4},