alternator-test: add tests for UpdateItem
alternator-test: add tests for UpdateItem

Add initial tests for UpdateItem. Only the features currently supported
by our code (only string attributes, only "PUT" action) are tested.

As usual, this test (like all others) was tested to pass on both DynamoDB
and Alternator.

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

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla