Test failure: no exception in UserManagerWillUseTokenProviderInstanceOverDefaults

[akoeplinger]

While looking at #57303 I discovered the UserManagerWillUseTokenProviderInstanceOverDefaults test from src/Identity/test/Identity.Test/UserManagerTest.cs doesn't actually throw a NotImplementedException, but no exception at all.

We didn't notice because we didn't await the Assert.ThrowsAsync.

Filing this issue for tracking.