tests/stress: Add test to verify the GC can trace nested objects.

tests/run-tests

@@ -366,6 +366,7 @@ def run_tests(pyb, tests, args, base_path="."):
         skip_tests.add('micropython/heapalloc_traceback.py') # because native doesn't have proper traceback info
         skip_tests.add('micropython/heapalloc_iter.py') # requires generators
         skip_tests.add('micropython/schedule.py') # native code doesn't check pending events
+        skip_tests.add('stress/gc_trace.py') # requires yield
 
     for test_file in tests:
         test_file = test_file.replace('\\', '/')

tests/stress/gc_trace.py

@@ -0,0 +1,17 @@
+# test that the GC can trace nested objects
+
+try:
+    import gc
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+# test a big shallow object pointing to many unique objects
+lst = [[i] for i in range(200)]
+gc.collect()
+print(lst)
+
+# test a deep object
+lst = [[[[[(i, j, k, l)] for i in range(3)] for j in range(3)] for k in range(3)] for l in range(3)]
+gc.collect()
+print(lst)