Add test for creating multiple conferences in one tox.

This triggers a code path in Persistent Group Chats that causes a memory
leak. I'm adding this test now, so that we don't merge PGC without fixing
the memory leak first.

CMakeLists.txt

@@ -498,6 +498,7 @@ endif()
 auto_test(TCP)
 auto_test(bootstrap)
 auto_test(conference)
+auto_test(conference_two)
 auto_test(crypto                        MSVC_DONT_BUILD)
 auto_test(dht                           MSVC_DONT_BUILD)
 auto_test(encryptsave)

auto_tests/check_compat.h

@@ -5,6 +5,7 @@
 
 #include <stdbool.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 #define START_TEST(name) static void name(void)
 #define END_TEST

auto_tests/conference_two_test.c

@@ -0,0 +1,30 @@
+// This test checks that we can create two conferences and quit properly.
+//
+// This test triggers a different code path than if we only allocate a single
+// conference. This is the simplest test possible that triggers it.
+#ifndef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+
+#include "../toxcore/tox.h"
+
+#include "check_compat.h"
+#include "helpers.h"
+
+int main(void)
+{
+    // Create toxes.
+    uint32_t id = 1;
+    Tox *tox1 = tox_new_log(nullptr, nullptr, &id);
+
+    // Create two conferences and then exit.
+    TOX_ERR_CONFERENCE_NEW err;
+    tox_conference_new(tox1, &err);
+    ck_assert_msg(err == TOX_ERR_CONFERENCE_NEW_OK, "failed to create conference 1: %d", err);
+    tox_conference_new(tox1, &err);
+    ck_assert_msg(err == TOX_ERR_CONFERENCE_NEW_OK, "failed to create conference 2: %d", err);
+
+    tox_kill(tox1);
+
+    return 0;
+}