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/vimscript/execute_spec.lua

@@ -114,7 +114,6 @@ describe('execute()', function()
 
   it('does not corrupt the command display #5422', function()
     local screen = Screen.new(70, 7)
-    screen:attach()
     feed(':echo execute("hi ErrorMsg")<CR>')
     screen:expect(
       [[
@@ -136,7 +135,6 @@ describe('execute()', function()
 
   it('places cursor correctly #6035', function()
     local screen = Screen.new(40, 6)
-    screen:attach()
     source([=[
       " test 1: non-silenced output goes as usual
       function! Test1()
@@ -263,7 +261,6 @@ describe('execute()', function()
   describe('{silent} argument', function()
     it('captures & displays output for ""', function()
       local screen = Screen.new(40, 5)
-      screen:attach()
       command('let g:mes = execute("echon 42", "")')
       screen:expect([[
       ^                                        |
@@ -287,7 +284,6 @@ describe('execute()', function()
 
     it('captures but does not display output for "silent"', function()
       local screen = Screen.new(40, 5)
-      screen:attach()
       command('let g:mes = execute("echon 42")')
       screen:expect([[
       ^                                        |