alternator test: de-duplicate some duplicate code
alternator test: de-duplicate some duplicate code

In test_streams.py we had some code to get a list of shards and iterators
duplicated three times. Put it in a function, shards_and_latest_iterators(),
to reduce this duplication.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20201006112421.426096-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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