testdrive: Fix default-storage-size parameter

Seen failing in https://buildkite.com/materialize/nightly/builds/11914#019664fd-9b58-4c90-a37c-ddadfba34360

Follow-up to MaterializeInc#32228

Author: def-

test/testdrive/github-6942.td

@@ -7,6 +7,8 @@
 # the Business Source License, use of this software will be governed
 # by the Apache License, Version 2.0.
 
+$ set-arg-default default-storage-size=1
+
 > DROP CLUSTER IF EXISTS gh_6942_cluster CASCADE;
 > CREATE CLUSTER gh_6942_cluster SIZE '${arg.default-storage-size}', REPLICATION FACTOR 1;
 

test/testdrive/webhook.td

@@ -8,6 +8,7 @@
 # by the Apache License, Version 2.0.
 
 $ set-arg-default replicas=1
+$ set-arg-default default-storage-size=1
 
 # Exercises Webhook sources.