test: exit from test child processes

Otherwise the child processes will continue, return to the test
main function, and try to run other test cases themselves:

<...>
/* test_namespace_get_leader */
PID hierarchy: 553438 ← 553459 ← 553460
/* test_detach_mount_namespace_harder */
/* test_detach_mount_namespace_harder */
/* test_detach_mount_namespace_harder */

Follow-up for 0b8b13324e

src/test/test-namespace.c

@@ -416,7 +416,11 @@ TEST(namespace_get_leader) {
                         ASSERT_TRUE(!pidref_equal(&pid2, &leader));
                         ASSERT_TRUE(!pidref_equal(&original, &leader));
                         ASSERT_TRUE(!pidref_equal(&grandparent, &leader));
+
+                        _exit(EXIT_SUCCESS);
                 }
+
+                _exit(EXIT_SUCCESS);
         }
 }