tests/micropython: Fully unlink nested list in extreme exc test.

To make sure there are no dangling references to the lists, and the GC can
reclaim heap memory.

Signed-off-by: Damien George <damien@micropython.org>

tests/micropython/extreme_exc.py

@@ -126,8 +126,8 @@ def main():
         )
     except Exception as er:
         e = er
-    lst[0][0] = None
-    lst = None
+    while lst:
+        lst[0], lst = None, lst[0]  # unlink lists to free up heap
     print(repr(e)[:10])
 
     # raise a deep exception with the heap locked