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/nested_function_spec.lua

@@ -2,7 +2,7 @@
 
 local helpers = require('test.functional.helpers')(after_each)
 local clear, insert = helpers.clear, helpers.insert
-local execute, expect, source = helpers.execute, helpers.expect, helpers.source
+local command, expect, source = helpers.command, helpers.expect, helpers.source
 
 describe('test_nested_function', function()
   setup(clear)
@@ -36,7 +36,7 @@ describe('test_nested_function', function()
       :  endfunction
       :  call s:{fn}()
       :endfunction]])
-    execute('call NestedFunc()')
+    command('call NestedFunc()')
 
     -- Assert buffer contents.
     expect([[