alternator-test: test_create_table_unsupported_names minor fix
alternator-test: test_create_table_unsupported_names minor fix

Check the expected error message to contain just ValidationException
instead of an overly specific text message from DynamoDB, so we aren't
so constraint in our own messages' wording.

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

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