Tests: IO, fixed a memory leak in the USDHierarchyIteratorTest

No functional changes to Blender.

source/blender/io/common/intern/abstract_hierarchy_iterator.cc

@@ -149,6 +149,12 @@ AbstractHierarchyIterator::AbstractHierarchyIterator(Depsgraph *depsgraph)
 
 AbstractHierarchyIterator::~AbstractHierarchyIterator()
 {
+  /* release_writers() cannot be called here directly, as it calls into the pure-virtual
+   * release_writer() function. By the time this destructor is called, the subclass that implements
+   * that pure-virtual function is already destructed. */
+  BLI_assert(
+      writers_.empty() ||
+      !"release_writers() should be called before the AbstractHierarchyIterator goes out of scope");
 }
 
 void AbstractHierarchyIterator::iterate_and_write()

source/blender/io/common/intern/abstract_hierarchy_iterator_test.cc

@@ -71,6 +71,7 @@ class TestingHierarchyIterator : public AbstractHierarchyIterator {
   }
   virtual ~TestingHierarchyIterator()
   {
+    release_writers();
   }
 
  protected: