Fixed errors and improve roster tests

tests/serialization.cpp

@@ -30,6 +30,10 @@ TEST(Serialize, Variant) {
             "xmlns=\"urn:ietf:params:xml:ns:xmpp-streams\"/></stream:error>\n"sv);
 }
 
+TEST(CheckFinalErrorPlace, Variant) {
+  ASSERT_EQ(std::variant_size<StreamError>::value, utempl::kTupleSize<StreamError>);
+}
+
 namespace tests::serialization {
 
 struct SomeStruct {