logictest: increase closed ts in buffered_writes further

We still see a RETRY_SERIALIZABLE error on COMMIT of a txn that reads
large blobs under race, so increase the closed ts interval even further
(beyond the test timeout).

Release note: None

Author: yuzefovich

pkg/sql/logictest/testdata/logic_test/buffered_writes

@@ -404,7 +404,7 @@ SELECT count(*) FROM uvw
 user host-cluster-root
 
 statement ok
-SET CLUSTER SETTING kv.closed_timestamp.target_duration = '30s'
+SET CLUSTER SETTING kv.closed_timestamp.target_duration = '99999s'
 
 user root