Convert TestCompatConfig test to use test fixtures

As this test is the only test in the apex package to use the
platform_compat_config module type it does not make sense to include
that in all the tests so instead this converts the test to use fixtures
so it can easily customize it with the additional module type.

Bug: 181070625
Test: m nothing
Change-Id: I56fda772ee336db6cfb677143aa28b1a18911bff