Expose canUseMemory

This lets CakeTestFixture be used to create an actual persistent table with InnoDB engine instead of MEMORY.

When using Lorenzo's [Fixturize plugin](https://github.com/lorenzo/cakephp-fixturize) to create tables from my test fixtures I found that the tables were being set to MEMORY engine which meant the data in them would disappear after a while.By exposing the canUseMemory flag it is possible to prevent this behavior.