Free the deep lazy import chain on the main thread in test_lazy_import

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.

Author: brittanyrey

Lib/test/test_lazy_import/__init__.py

@@ -318,6 +318,9 @@ def test_deeply_dotted_name(self):
         ns = {"__builtins__": {"__lazy_import__": __lazy_import__,
                                "__import__": lambda *args: m}}
         exec("lazy import " + ".".join(["m"] * 10_000) + " as x", ns)
+        # Free the chain in the main thread: in a thread with a small C stack,
+        # deallocations deferred by the trashcan are never run.
+        self.addCleanup(ns.clear)
         with contextlib.suppress(RecursionError):
             ns["x"].resolve()