alternator-test: exhaustive tests for GSI
alternator-test: exhaustive tests for GSI

This patch adds what is hopefully an exhaustive test suite for the
global secondary indexing (GSI) feature, and all its various
complications and corner cases of how GSIs can be created, deleted,
named, written, read, and more (the tests are heavily documented to
explain what they are testing).

All these tests pass on DynamoDB, and fail on Alternator, so they are
marked "xfail". As we develop the GSI feature in Alternator piece by
piece, we should make these tests start to pass.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20190708160145.13865-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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