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/036_regexp_character_classes_spec.lua

@@ -1,7 +1,7 @@
 -- Test character classes in regexp using regexpengine 0, 1, 2.
 
 local helpers = require('test.functional.helpers')(after_each)
-local clear, execute, expect = helpers.clear, helpers.execute, helpers.expect
+local clear, command, expect = helpers.clear, helpers.command, helpers.expect
 local source, write_file = helpers.source, helpers.write_file
 
 local function sixlines(text)
@@ -14,7 +14,7 @@ end
 
 local function diff(text, nodedent)
   local fname = helpers.tmpname()
-  execute('w! '..fname)
+  command('w! '..fname)
   helpers.wait()
   local data = io.open(fname):read('*all')
   if nodedent then
@@ -45,7 +45,7 @@ describe('character classes in regexp', function()
   end)
   before_each(function()
     clear()
-    execute('e test36.in')
+    command('e test36.in')
   end)
   teardown(function()
     os.remove('test36.in')