Add getStartingAnchorRoot to MockAnchorStateRegistry

[leopoldjoy]

AggregateVerifier calls getStartingAnchorRoot() on the AnchorStateRegistry when initialising the first dispute game (parentIndex = uint32.max). The MockAnchorStateRegistry was missing this function, causing game creation to revert. Added in #201 when AggregateVerifier switched from getAnchorRoot() to getStartingAnchorRoot(), but the mock was not updated.

[cb-heimdall]

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