test/cql-pytest: avoid spurious guardrail warnings

All cql-pytest tests use one node, and unsuprisingly most use RF=1.
By default, as part of the "guardrails" feature, we print a warning
when creating a keyspace with RF=1. This warning gets printed on
every cql-pytest run, which creates a "boy who cried wolf" effect
whereby developers get used to seeing these warnings, and won't care
if new warnings start appearing.

The fix is easy - in run.py start Scylla with minimum-replication-factor-
warn-threshold set to -1 instead of the default 3.

Note that we do have cql-pytest tests for this guardrail, but those don't
rely on the default setting of this variable (they can't, cql-pytest
tests can also be run on a Scylla instance run manually by a developer).
Those tests temporarily set the threshold during the test.

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

Closes scylladb/scylladb#17274

test/cql-pytest/run.py

@@ -325,6 +325,8 @@ def run_scylla_cmd(pid, dir):
         '--shutdown-announce-in-ms', '0',
         '--maintenance-socket', 'workdir',
         '--service-levels-interval-ms', '500',
+        # Avoid unhelpful "guardrails" warnings
+        '--minimum-replication-factor-warn-threshold', '-1',
         ], env)
 
 # Same as run_scylla_cmd, just use SSL encryption for the CQL port (same