LongBTreeTest times out after btree improvements from CASSANDRA-15510

Happening in both ci-cassandra.a.o and circleci.

LongBTreeTest is timing out on 4.0, 4.1, trunk branches.

Started back in mid April (https://github.com/apache/cassandra/commit/018c8e0d5e and https://github.com/apache/cassandra/commit/596daeb7f08).

Nightlies shows when the failures started, evident by the 'jdk=jdk_1.8_latest,label=cassandra,split=7/' subfolder missing in the following…

• https://nightlies.apache.org/cassandra/trunk/Cassandra-trunk-test-burn/1254/Cassandra-trunk-test-burn/
• https://nightlies.apache.org/cassandra/cassandra-4.0/Cassandra-4.0-test-burn/343/