asyncio: Make sure sys.set_coroutine_wrapper is called *only* when loop is running.

Previous approach of installing coroutine wrapper in loop.set_debug() and
uninstalling it in loop.close() was very fragile.  Most of asyncio tests
do not call loop.close() at all.  Since coroutine wrapper is a global
setting, we have to make sure that it's only set when the loop is
running, and is automatically unset when it stops running.

Issue #24017.