Fix PeriodicTimer_ActiveOperations_TimerRooted test (#68805)

Fix PeriodicTimer_ActiveOperations_TimerRooted test (#68805)

There are two problems with this test
1. `WaitForNextTickAsync` may return a synchronously completed task, in
   which case it does not root the timer, causing our first
   `WaitForTimerToBeCollected` to fail because the timer was collected.
   This problem is easily reproduced by adding a small sleep after
   constructing the `PeriodicTimer` in `Create`, and I believe it is the
   cause of #59542.
2. There is no guarantee that the timer is not still rooted after the
   wait finishes because the returned `ValueTask<bool>` may be keeping
   it alive (although, it seems unlikely Roslyn will extend the lifetime across the await like this).
   Fixed by wrapping in another NoInlining method.

Fix #59542