[5.x] Fix test state issues around sites cache

[jesseleite]

The problem

We were intermittently seeing test errors related to sites, but only when running certain tests with no cache state. For example, this would fail...

rm -rf vendor
composer update
phpunit --filter SubstitutesEntryForLivePreviewTest

...but only on the first run. That test would pass on subsequent runs, or when running the whole suite, because sites state had been built up already.

The solution

The way our Sites class works, it caches $sites as a property cache, and has some fallback logic as well. We intentionally set up a default sites.yaml in our TestCase class, so we should just be making sure that file gets set up early enough for when the facade runs.

Note

We also have a bit more work to do around places like this...

cms/src/Stache/Repositories/EntryRepository.php

Line 75 in 297ed95

$site = $site ?? $this->stache->sites()->first();

Which uses a separate sites cache here...

cms/src/Stache/ServiceProvider.php

Line 45 in 297ed95

$stache->sites(Site::all()->keys()->all());

Which we think should just be a Site::default() call, using the Sites class property cache as a single source of truth, as we do everywhere else in Statamic. Jason has a PR incoming to make this change for v6 👌