[MIMIR ISSUE 446] test: Implement e2e tests for the stake state

[jihye-woo]

resolve #446

특정 address에 대해 Mimir와 Headless 서비스에서 각각 조회한 Stake의 StartedBlockIndex가 같은지 비교하는 테스트 코드를 작성합니다

Additional FIXME

• stake.receivedBlockIndex is almost alway 0 #448
• Does not handle case when stake data is Bencodex.Types.Null #449

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