test/rbd_mirror: clear Namespace::s_instance at the end of a test

TestMockPoolReplayer.Namespaces and NamespacesError tests leave behind
a dangling pointer to a stack-allocated MockNamespace which leads to an
easily reproducible use-after-free and segfault when tests are shuffled.

Fixes: https://tracker.ceph.com/issues/70041
Signed-off-by: Ilya Dryomov <[email protected]>

Author: idryomov

src/test/rbd_mirror/test_mock_PoolReplayer.cc

@@ -56,9 +56,15 @@ class Namespace<MockTestImageCtx> {
   }
 
   Namespace() {
+    ceph_assert(s_instance == nullptr);
     s_instance = this;
   }
 
+  ~Namespace() {
+    ceph_assert(s_instance == this);
+    s_instance = nullptr;
+  }
+
   void add(const std::string &name) {
     std::lock_guard locker{m_lock};