Investigate setting the apcu cache size lower

Also, cashe item lifetimes might prevent flushing.

We also share FileCache apcu entries between test runs due to a cache key optimisation specifically for test environments, so that should also be limiting how much is in apcu.

Maybe we can find out what the actual usage is by adding https://www.php.net/manual/en/apcuiterator.gettotalsize.php - would need to do it both in core somewhere and in run-tests.sh since the cache won't be shared between the two then figure out a way to output it.