Add option to *not* resize the object cache

[jamadden]

Currently, the read tests always maximizes the object cache to a value larger than the total number of objects, ignoring the value specified in the database configuration. This ensures that we're using the storage in a predictable way, but it masks the effect of differing object cache sizes.

Especially after #23, it may be very useful to be able to test and compare different object cache sizes for a given workload.

It may also be useful to test and compare different object cache sizes when the object cache implementation itself is being changed (cf. zopefoundation/persistent#45).