The tests in test/cql-pytest/test_guardrail_replication_strategy.py
are for a Scylla-only feature that doesn't exist in Cassandra, so
obviously they all fail on Cassandra. Let's mark them all as
scylla_only.
We use an autouse fixture to automatically mark all tests in this file
as scylla-only, instead of marking each one separately.
Signed-off-by: Nadav Har'El <nyh@scylladb.com>