Some tests need to change settings, but since MemoryCacheSettingsRepository caches settings in-memory, those changes aren't reflected. The new `purgeSettingsCache` removes it from the container, eliminating that cache.
For UserTest, we also need to regenerate the display name driver, since that's set statically on boot, before we'll get a change to clear the settings cache.
Previously, the `prepareDatabase` method would directly modify the database, booting the app in the process. This would prevent any extenders from being applied, since `->extend()` has no effect once the app is booted.
Since the new implementation of `prepareDatabase` simply registers seed data to be applied during app boot, the workaround of sticking this seed data into `prepDb` is no longer necessary, and seed data common to all test cases in a class can be provided in `setUp`.
When needed, app boot is explicitly triggered in individual test cases by calling `$this->app()`.
