test/alternator: add tests for expression length limits
test/alternator: add tests for expression length limits

The DynamoDB documentation
https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Limits.html
describes several hard limits on the size of the size of expressions
(ProjectionExpression, ConditionExpression, UpdateExpression,
FilterExpression) and various elements they contain.

In this patch we begin testing those limits with a comprehensive test for
the *length* of each of these four expressions: we test that lengths up to
(and including) 4096 bytes are allowed but longer expressions are rejected.
We also add TODOs for additional documented limits that should be tested
in the future.

Currently, this test passes on DynamoDB but xfails on Alternator because
Alternator does *not* enforce any limits on the expression length. I don't
think this is a real problem, and we may consider keeping it this way,
but we should at least be aware that this difference exists and an
xfailing test will remind us.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20210810081948.2012120-2-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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