alternator-test: add tests for UpdateItem's AttributeUpdates DELETE and ADD
alternator-test: add tests for UpdateItem's AttributeUpdates DELETE and ADD

We have not yet implemented the DELETE-with-value and ADD operations in
UpdateItem's old-style "AttributeUpdates" parameter - see issue #5864
and issue #5893, respectively

This patch include comprehensive tests for both features. The new tests
pass on DynamoDB, but currently xfails on Alternator - until these
features will be implemented.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20200225105546.25651-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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