Avoid accidentally sharing preparers slice across factories

Previously, there was a bug that caused tests which ran successfully on
their own to fail when run together in parallel. They each extended the
same factory and ended up sharing the preparers slice which meant that
they overwrote each other's preparers causing the tests to fail.

This change fixes that by creating a new slice for each factory.

Bug: 181070625
Test: m nothing
Change-Id: If340c125c5b03a9d5c36a59ff4da4ec189808f9b