[Code health] Allow instrumented test fake state to be initialized explicitly

Test fakes currently need to be set statically so that the instrumented tests have the correct initial state. These fakes are shared with unit tests, and shouldn't set initial state which might influence other test behavior or add complexity and indirection to tests.

Instead of reusing static constant test data, we should expose helper methods to build and set default state, but only relevant state in tests:
https://testing.googleblog.com/2018/02/testing-on-toilet-cleanly-create-test.html