functests: Test for error conditions

During testing found the following bugs:

1. msgpack-gen.lua script is completely unprepared for Float values either in 
   return type or in arguments. Specifically:

   1. At the time of writing relevant code FLOAT_OBJ did not exist as well as 
      FLOATING_OBJ, but it would be used by msgpack-gen.lua should return type 
      be Float. I added FLOATING_OBJ macros later because did not know that 
      msgpack-gen.lua uses these _OBJ macros, otherwise it would be FLOAT_OBJ.
   2. msgpack-gen.lua should use .data.floating in place of .data.float. But it 
      did not expect that .data subattribute may have name different from 
      lowercased type name.

2. vim_replace_termcodes returned its argument as-is if it receives an empty 
   string (as well as _vim_id*() functions did). But if something in returned 
   argument lives in an allocated memory such action will cause double free: 
   once when freeing arguments, then when freeing return value. It did not cause 
   problems yet because msgpack bindings return empty string as {NULL, 0} and 
   nothing was actually allocated.
3. New code in msgpack-gen.lua popped arguments in reversed order, making lua 
   bindings’ signatures be different from API ones.

src/nvim/msgpack_rpc/helpers.c

@@ -117,7 +117,7 @@ bool msgpack_rpc_to_object(const msgpack_object *const obj, Object *const arg)
       case MSGPACK_OBJECT_FLOAT: {
         STATIC_ASSERT(sizeof(Float) == sizeof(cur.mobj->via.f64),
                       "Msgpack floating-point size does not match API integer");
-        *cur.aobj = FLOATING_OBJ(cur.mobj->via.f64);
+        *cur.aobj = FLOAT_OBJ(cur.mobj->via.f64);
         break;
       }
 #define STR_CASE(type, attr, obj, dest, conv) \