Fix prebuilt mutator ordering in tests

Previously, the prebuilt mutators were added by the
cc.RegisterRequiredBuildComponentsForTest() function as a convenience
but unfortunately it lead to some of the mutators being in a different
order in the tests than in the normal build.

This change:
* Extracts the RegisterPrebuiltMutators() call from
  cc.RegisterRequiredBuildComponentsForTest()
* Makes sure that the prebuilt mutators are registered before the
  visibility gatherer and enforcer mutators.

Bug: 162505935
Test: m nothing
Change-Id: I7d959b558200b502f0a5e4653c41ea01414e142a