The main object cache is disabled during testing. Some integration tests
need it though. This provides a clean way to enable it, to replace the hacks
that were used so far.
Note that we may want to enable the main cache during testing soon. When
that happens, this method is still useful to disable the cache in certain
tests, and to set a specific cache instance.
Change-Id: I04ae1bf1b6b2c8f6310acd2edf89459d01a9c870
bf092744c9