Keep spack.store.store and spack.store.db consistent in unit tests
* Remove calls to monkeypatch for spack.store.store and spack.store.db:
tests that used these called one or the other, which lead to
inconsistencies (the tests passed regardless but were fragile as a
result)
* Fixtures making use of monkeypatch with mock_store now use the
updated use_store function, which sets store.store and store.db
consistently
* subprocess_context.TestState now transfers the serializes and
restores spack.store.store (without the monkeypatch changes this
would have created inconsistencies)
f2e3edf6db