test-dns-question: use ASSERT_OK_POSITIVE/ZERO() rather than ASSERT_TRUE/FALSE()

Similar to 100a98960ac2ea8d1d7297e35960cd9c33b0ce19.

E.g. dns_question_contains_key() may return negative errno, hence we
should not use ASSERT_TRUE/FALSE() for the function.

This also has bunch of cleanups:
- call functions in ASSERT_NOT_NULL(),
- add short comments for constant function arguments,
- merge several test cases,
- use memstream, rather than temporal files.

Fixes CID#1619185.

src/shared/tests.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <unistd.h>
+
 #include "errno-list.h"
 #include "errno-util.h"
 #include "forward.h"