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/api/buffer_spec.lua

@@ -5,7 +5,7 @@ local curbufmeths, ok = helpers.curbufmeths, helpers.ok
 local funcs = helpers.funcs
 local request = helpers.request
 local exc_exec = helpers.exc_exec
-local execute = helpers.execute
+local feed_command = helpers.feed_command
 local insert = helpers.insert
 local NIL = helpers.NIL
 local meth_pcall = helpers.meth_pcall
@@ -246,7 +246,7 @@ describe('api/buf', function()
     end)
 
     it("set_line on alternate buffer does not access invalid line (E315)", function()
-      execute('set hidden')
+      feed_command('set hidden')
       insert('Initial file')
       command('enew')
       insert([[
@@ -257,7 +257,7 @@ describe('api/buf', function()
       The
       Other
       Buffer]])
-      execute('$')
+      feed_command('$')
       local retval = exc_exec("call nvim_buf_set_lines(1, 0, 1, v:false, ['test'])")
       eq(0, retval)
     end)