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/api/window_spec.lua

@@ -170,7 +170,6 @@ describe('API/win', function()
 
     it('updates the screen, and also when the window is unfocused', function()
       local screen = Screen.new(30, 9)
-      screen:attach()
 
       insert('prologue')
       feed('100o<esc>')
@@ -281,7 +280,6 @@ describe('API/win', function()
 
     it('updates cursorline and statusline ruler in non-current window', function()
       local screen = Screen.new(60, 8)
-      screen:attach()
       command('set ruler')
       command('set cursorline')
       insert([[
@@ -314,7 +312,6 @@ describe('API/win', function()
 
     it('updates cursorcolumn in non-current window', function()
       local screen = Screen.new(60, 8)
-      screen:attach()
       command('set cursorcolumn')
       insert([[
         aaa
@@ -857,7 +854,6 @@ describe('API/win', function()
     it('with two diff windows', function()
       local X = api.nvim_get_vvar('maxcol')
       local screen = Screen.new(45, 22)
-      screen:attach()
       exec([[
         set diffopt+=context:2 number
         let expr = 'printf("%08d", v:val) .. repeat("!", v:val)'
@@ -975,7 +971,6 @@ describe('API/win', function()
     it('with wrapped lines', function()
       local X = api.nvim_get_vvar('maxcol')
       local screen = Screen.new(45, 22)
-      screen:attach()
       exec([[
         set number cpoptions+=n
         call setline(1, repeat([repeat('foobar-', 36)], 3))
@@ -2557,7 +2552,6 @@ describe('API/win', function()
 
     it('updates statusline when moving bottom split', function()
       local screen = Screen.new(10, 10)
-      screen:attach()
       exec([[
         set laststatus=0
         belowright split