Most functions of the new storage for raft groups for strongly
consistent tables are the same as for the system raft table
storage, so we reuse the tests for them to test the new storage.
We add additional tests for checking the new raft groups partitioner
and sharder, and for verifying that writes using storages for different
shards do not affect the data read on different shards.
We also add a test for checking the snapshot_descriptor present after
the storage bootstrap - for both system and strongly consistent storages
we check that the storage contains the initial descriptor.
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