test: Add return 0 to main() function (even it is not strictly necessary)

src/journal/test-audit-type.c

@@ -22,4 +22,5 @@ static void test_audit_type(void) {
 
 int main(int argc, char **argv) {
         test_audit_type();
+        return 0;
 }

src/libsystemd-network/test-sd-dhcp-lease.c

@@ -87,4 +87,5 @@ int main(int argc, char *argv[]) {
         test_dhcp_lease_parse_search_domains_no_data();
         test_dhcp_lease_parse_search_domains_loops();
         test_dhcp_lease_parse_search_domains_wrong_len();
+        return 0;
 }

src/network/test-network.c

@@ -253,4 +253,5 @@ int main(void) {
         test_network_get(manager, loopback);
 
         assert_se(manager_rtnl_enumerate_links(manager) >= 0);
+        return 0;
 }