check delta chat group creation

test-delta-groups.c

// this snippet does not care about freeing allocated objects!
dc_context_t* context = dc_context_new(NULL, NULL, NULL);
dc_delete_file(context,  "/tmp/somefile");
dc_open(context, "/tmp/somefile", NULL);

int contact_id1 = dc_create_contact(context, "some1", "some1@hello.com");
int contact_id2 = dc_create_contact(context, "some2", "some2@hello.com");
int chat_id     = dc_create_group_chat(context, 0, "title1");

dc_add_contact_to_chat(context, chat_id, contact_id1);
dc_add_contact_to_chat(context, chat_id, contact_id2);

dc_array_t* chat_contacts = dc_get_chat_contacts(context, chat_id);
for (size_t i=0; i<dc_array_get_cnt(chat_contacts); i++ ) {
	// prints `10 - 11 - 1 - ` the 1 is SELF that is always added to group chats
	printf("%i - ", dc_array_get_id(chat_contacts, i));
}
printf("\n");

dc_array_t* all_contacts = dc_get_contacts(context, 0, NULL);
for (size_t i=0; i<dc_array_get_cnt(all_contacts); i++ ) {
	// prints `10 - 11 - ` - DC_GCL_ADD_SELF is not added to flags
	printf("%i - ", dc_array_get_id(all_contacts, i));
}

see deltachat/deltachat-core#294 (review)