gh-159538: Fix deep chain leak in test_deeply_dotted_name (#159576)

test_deeply_dotted_name() builds a 10,000-deep chain of lazy import
placeholders in a thread with a 256 KiB C stack.  Freeing the chain
there defers most deallocations to the trashcan, which only drains them
once the stack margin recovers.  With a stack that small the margin
never does, so the deferred objects were leaked when the thread exited,
which -R reports as a reference leak on platforms with larger frames
such as PPC64LE.

Clear the namespace in a cleanup, which runs on the main thread.