Support test fixtures in sh package

Restructures the sh package test setup code to create FixturePreparer
instances for setting up a test fixture and converts some tests to
use it.

The goal with this change is not to switch all the sh tests over to
directly using the new model but instead to ensure that the majority of
the sh tests run with the new model, to allow existing tests to easily
switch to the new model when needed and to allow dependent packages to
be switched to the new model.

Bug: 181070625
Test: m nothing
Change-Id: Ib2c6ac2842d6fafde5663d3ee63c3f934913a00e