fix(tests): needing two calls to setup a screen is cringe

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.

test/functional/ui/diff_spec.lua

@@ -53,7 +53,6 @@ describe('Diff mode screen', function()
     feed('<c-w>w:diffthis<cr><c-w>w')
 
     screen = Screen.new(40, 16)
-    screen:attach()
   end)
 
   it('Add a line in beginning of file 2', function()
@@ -1172,7 +1171,6 @@ end)
 it('win_update redraws lines properly', function()
   local screen
   screen = Screen.new(50, 10)
-  screen:attach()
   screen:set_default_attr_ids({
     [1] = { bold = true, foreground = Screen.colors.Blue1 },
     [2] = { foreground = Screen.colors.Grey100, background = Screen.colors.Red },
@@ -1250,7 +1248,6 @@ end)
 -- oldtest: Test_diff_rnu()
 it('diff updates line numbers below filler lines', function()
   local screen = Screen.new(40, 14)
-  screen:attach()
   exec([[
     call setline(1, ['a', 'a', 'a', 'y', 'b', 'b', 'b', 'b', 'b'])
     vnew
@@ -1310,7 +1307,6 @@ end)
 -- oldtest: Test_diff_with_scroll_and_change()
 it('Align the filler lines when changing text in diff mode', function()
   local screen = Screen.new(40, 20)
-  screen:attach()
   exec([[
     call setline(1, range(1, 15))
     vnew
@@ -1376,7 +1372,6 @@ end)
 
 it("diff mode doesn't restore invalid 'foldcolumn' value #21647", function()
   local screen = Screen.new(60, 6)
-  screen:attach()
   eq('0', api.nvim_get_option_value('foldcolumn', {}))
   command('diffsplit | bd')
   screen:expect([[
@@ -1389,7 +1384,6 @@ end)
 
 it("'relativenumber' doesn't draw beyond end of window in diff mode #29403", function()
   local screen = Screen.new(60, 12)
-  screen:attach()
   command('set relativenumber')
   feed('10aa<CR><Esc>gg')
   command('vnew')
@@ -1433,7 +1427,6 @@ it('diff mode works properly if file contains NUL bytes vim-patch:8.2.3925', fun
   screen:add_extra_attr_ids {
     [100] = { foreground = Screen.colors.Blue, bold = true, background = Screen.colors.Red },
   }
-  screen:attach()
   exec([[
     call setline(1, ['a', 'b', "c\n", 'd', 'e', 'f', 'g'])
     vnew
@@ -1510,7 +1503,6 @@ end)
 -- oldtest: Test_diff_breakindent_after_filler()
 it("diff mode draws 'breakindent' correctly after filler lines", function()
   local screen = Screen.new(45, 8)
-  screen:attach()
   exec([[
     set laststatus=0 diffopt+=followwrap breakindent breakindentopt=min:0
     call setline(1, ['a', '  ' .. repeat('c', 50)])
@@ -1562,7 +1554,6 @@ it('diff mode overlapped diff blocks will be merged', function()
   ]])
 
   local screen = Screen.new(35, 20)
-  screen:attach()
   command('set winwidth=10 diffopt=filler,internal')
 
   command('args Xdifile1 Xdifile2 | vert all | windo diffthis')
@@ -2058,7 +2049,6 @@ end)
 -- oldtest: Test_diff_topline_noscroll()
 it('diff mode does not scroll with line("w0")', function()
   local screen = Screen.new(45, 20)
-  screen:attach()
   exec([[
     set scrolloff=5
     call setline(1, range(1,60))