Specify auth superuser name for tests

[sylwiaszunejko]

Recently scylladb started to rely on the options "--auth-superuser-name" and "--auth-superuser-salted-password" to ensure that a cassandra/cassandra user exists for tests - without those options a default superuser no longer exists (see: scylladb/scylladb#27215).

This caused the driver matrix tests running against scylladb/master to fail.

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