Support testing by mocking MockRedis#evalsha

[vu-hoang-kaligo]

Context

Application uses MockRedis will have test cases related to Idempotency cache failing due to the Idempotency::Cache#evalsha method always return nil, which causes the release_lock logic to always fail:

    def release_lock(fingerprint, acquired_lock)
      with_redis do |r|
        lock_released = r.evalsha(COMPARE_AND_DEL_SCRIPT_SHA, keys: [lock_key(fingerprint)], argv: [acquired_lock])
		# lock_released is always `nil` which will differ from `acquired_lock` value
        raise LockConflict if lock_released != acquired_lock

As such, I've created a new helper in this PR to override the evalsha implementation of MockRedis to work similar to the LUA script logic