alternator-test: improve test_update_expression_cannot_modify_key
alternator-test: improve test_update_expression_cannot_modify_key

The test test_update_expression_cannot_modify_key() verifies that an
update expression cannot modify one of the key columns. The existing
test only tried the SET and REMOVE actions - this patch makes the
test more complete by also testing the ADD and DELETE actions.

This patch also makes the expected exception more picky - we now
expect that the exception message contains the word "key" (as it,
indeed, does on both DynamoDB and Alternator). If we get any other
exception, there may be a problem.

The test passed before this patch, and passes now as well - it's just
stricter now.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20190829135650.30928-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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