Create cassandra superuser manually in test env

[Copilot]

Starting from Scylla 2026.2, the cassandra superuser is no longer created by default on cluster startup and must be configured manually via the maintenance socket. This PR adapts the test environment to match the new behavior while remaining backward-compatible with older Scylla versions.

chore(testing): create cassandra superuser manually

• Enables maintenance_socket: workdir in testing/scylla/config/scylla.yaml, exposing a Unix socket at /var/lib/scylla/cql.m that accepts unauthenticated, no-TLS CQL connections before any users exist.
• After cluster startup in testing/Makefile, connects to that socket and creates the cassandra superuser explicitly:

  cqlsh /var/lib/scylla/cql.m --cqlshrc=/dev/null \
    -e "CREATE ROLE cassandra WITH PASSWORD = 'cassandra' AND LOGIN = true AND SUPERUSER = true" || true

  The || true ensures backward compatibility — on Scylla < 2026.2 where the role already exists by default, the failure is silently ignored.

chore(treewide): stabilize CI

• TestServiceStorageIntegration: sort clusters by ID before comparison to fix non-deterministic ordering flakiness.
• TestRestoreGetTargetUnitsViewsIntegration: replace exact TombstoneGC value comparison with a non-empty check, as the default mode changed between Scylla versions.
• TestServiceRepairIntegration/repair_ignore_hosts: increase timeout from shortWait to longWait to address flakiness on tablet keyspaces.

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