Update tests for set and check sharding function with cdata

There are at least two ways for setting metadata for Lua cdata object:
with and without ffi.new(). I found second way more clear because first
one should use a function with placeholder instead of first argument and
it looks a bit opaque for me.

Follows up #71

Author: ligurio

test/check_sharding_metadata_test.lua

@@ -145,11 +145,11 @@ function g.test_user_sharding_func_cdata_call()
             int data;
         } test_check_struct_t;
     ]]
-
-    local test_check_struct = ffi.metatype('test_check_struct_t', {
+    ffi.metatype('test_check_struct_t', {
         __call = function(_, key) return key end
     })
-    rawset(_G, user_sharding_func_name, test_check_struct())
+    local test_check_struct = ffi.new('test_check_struct_t')
+    rawset(_G, user_sharding_func_name, test_check_struct)
     g.space.sharding_func = user_sharding_func_name
 
     local res, err = ddl.check_schema(g.schema)

test/set_sharding_metadata_test.lua

@@ -424,10 +424,11 @@ function g.test_ddl_user_sharding_func_cdata_call()
         } test_set_struct_t;
     ]]
 
-    local test_set_struct = ffi.metatype('test_set_struct_t', {
+    ffi.metatype('test_set_struct_t', {
         __call = function(_, key) return key end
     })
-    rawset(_G, user_sharding_func_name, test_set_struct())
+    local test_set_struct = ffi.new('test_set_struct_t')
+    rawset(_G, user_sharding_func_name, test_set_struct)
     space_one.sharding_func = user_sharding_func_name
 
     local schema = {