Support running bp2build tests in fixtures

Previously, the fixture test infrastructure was hard coded to
initialize itself in preparation for invoking the PrepareBuildActions()
method. That meant it could not be used for testing the bp2build and
apiBp2build modes which required different initialization and called a
different method.

This change extracts that behavior into a FixtureTestRunner and adds an
implementation that allows it to test the above two modes. It then uses
that to implement the runBp2BuildTestCaseWithSetup method.

The TestPrebuiltLibraryAdditionalAttrs was the only test which broke as
it supplied an invalid path to the MockFS (it does not support using a
trailing / to represent an empty build directory). So, it was modified
to add a couple of files inside those directories instead.

Test: m nothing
Change-Id: I6798a4f761160af8d1bfed81d46de9628bda3eb9