Fix CountdownEvent.Wait to respect underlying event's status (dotnet/corefx#35024)

Fix CountdownEvent.Wait to respect underlying event's status (dotnet/corefx#35024)

A common pattern with CountdownEvent is:
```C#
ce.Reset(count);
.. // launch count operations that'll all call ce.Signal()
ce.Wait();
```
and to use that repeatedly.  Reset is explicitly not a thread-safe operation, but in a pattern like this, it should be acceptable as long as the only operations signaling the event are those launched here, since Wait should only return when they've all completed.  However, CountdownEvent.Wait is currently checking CountdownEvent.IsSet, which returns true if the count has reached 0, and the CountdownEvent.Signal method first decrements the count and the sets the underlying event if it reaches 0.  That means there's a window where the count is 0 but the event hasn't been set, which means if Wait is called in that window, it could return successfully even though an operation is still in flight invoking Signal, which then in turn is problematic as that could race with a subsequent call to Reset.

This fixes that by checking this._event.IsSet instead of this.IsSet.

Commit migrated from https://github.com/dotnet/corefx/commit/66403fb0e4c9e3c05e23db23c04742b8a50dbe0a