test/alternator: check correct error for unknown operation
test/alternator: check correct error for unknown operation

Add a short test verifying that Alternator responds with the correct
error code (UnknownOperationException) when receiving an unknown or
unsupported operation.

The test passes on both AWS and Alternator, confirming that the behavior
is the same.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20211206125710.1153008-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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