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/buffer_spec.lua

@@ -125,7 +125,6 @@ describe('api/buf', function()
 
     it('cursor position is maintained consistently with viewport', function()
       local screen = Screen.new(20, 12)
-      screen:attach()
 
       local lines = { 'line1', 'line2', 'line3', 'line4', 'line5', 'line6' }
       local buf = api.nvim_get_current_buf()
@@ -211,7 +210,6 @@ describe('api/buf', function()
       local screen
       before_each(function()
         screen = Screen.new(20, 12)
-        screen:attach()
         api.nvim_buf_set_lines(
           0,
           0,
@@ -735,7 +733,6 @@ describe('api/buf', function()
 
     it("set_lines of invisible buffer doesn't move cursor in current window", function()
       local screen = Screen.new(20, 5)
-      screen:attach()
 
       insert([[
         Who would win?
@@ -1689,7 +1686,6 @@ describe('api/buf', function()
 
     it('correctly marks changed region for redraw #13890', function()
       local screen = Screen.new(20, 5)
-      screen:attach()
 
       insert([[
       AAA
@@ -1742,7 +1738,6 @@ describe('api/buf', function()
       local screen
       before_each(function()
         screen = Screen.new(20, 12)
-        screen:attach()
         api.nvim_buf_set_lines(
           0,
           0,