gh-117300: Use stop the world to make `sys._current_frames` and `sys._current_exceptions` thread-safe. (#117301)

This adds a stop the world pause to make the two functions thread-safe
when the GIL is disabled in the free-threaded build.

Additionally, the main test thread may call `sys._current_exceptions()` as
soon as `g_raised.set()` is called. The background thread may not yet reach
the `leave_g.wait()` line.