functests: Replace execute with either command or feed_command

Hope this will make people using feed_command less likely: this hides bugs.
Already found at least two:

1. msgpackparse() will show internal error: hash_add() in case of duplicate
   keys, though it will still work correctly. Currently silenced.
2. ttimeoutlen was spelled incorrectly, resulting in option not being set when
   expected. Test was still functioning somehow though. Currently fixed.

test/functional/legacy/autochdir_spec.lua

@@ -1,7 +1,7 @@
 local lfs = require('lfs')
 local helpers = require('test.functional.helpers')(after_each)
 local clear, eq = helpers.clear, helpers.eq
-local eval, execute = helpers.eval, helpers.execute
+local eval, command = helpers.eval, helpers.command
 
 describe('autochdir behavior', function()
   local dir = 'Xtest-functional-legacy-autochdir'
@@ -17,9 +17,9 @@ describe('autochdir behavior', function()
 
   -- Tests vim/vim/777 without test_autochdir().
   it('sets filename', function()
-    execute('set acd')
-    execute('new')
-    execute('w '..dir..'/Xtest')
+    command('set acd')
+    command('new')
+    command('w '..dir..'/Xtest')
     eq('Xtest', eval("expand('%')"))
     eq(dir, eval([[substitute(getcwd(), '.*[/\\]\(\k*\)', '\1', '')]]))
   end)