The purpose of the test was to ensure that the SX (update) mode of
index tree and buffer page latches are being used.

The test has become unstable, possibly due to changes related to
buf_pool.mutex and buf_pool.page_hash, or to the use of MDL in the
purge of transaction history.

In 10.6, the test depends on instrumentation that was refactored
or removed in MDEV-24142.

The use of different latching modes can better be indirectly observed
through high-concurrency benchmarks. For MDEV-14637, a performance test
was conducted where the finer-grained latching and
BTR_CUR_FINE_HISTORY_LENGTH were removed. It caused a 20% performance
regression for UPDATE and somewhat smaller for INSERT.

Any new problem with latching granularity should be easily caught by
performance testing, or by stress tests with Random Query Generator.