Update MockProvider

[shweaver-MSFT]

Describe the bug

The MockProvider is not working because the underlying endpoint has changed.

• Is this bug a regression in the toolkit? If so, what toolkit version did you last see it work:

Steps to Reproduce

Steps to reproduce the behavior:
Use the MockProvider, observe nothing works.

Expected behavior

The MockProvider should enable access to some mock Graph data from a specialized endpoint.

Additional context

See MGT as a reference: https://github.com/microsoftgraph/microsoft-graph-toolkit/pull/1198/files

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