make caching of tables to truncate optional

When creating (and dropping) tables during tests, the caching behavior where the list of tables that need to be truncated is cached can lead to problems when a table makes it into the list but is later dropped. This commit makes that caching (introduced in #130) optional.

Note: I often have tests that test additions to ActiveRecord that I test on dedicated test models for which I then create and drop the tables during the test so that I don't have to rely on an application model to test sth. that's actually unrelated to the application logic.