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()`.

Before transactions, each test class would need to explicitly state starting state for permissions, which made the initial permission configuration somewhat arbitrary. Now, we might as well use the initial state of the default installation.

One of the User show_test tests has been commented out until

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.

Under InnoDB, database entries created in transactions are not processed by fulltext indexes until the transaction is committed. To work around this, cases that test fulltext search have been split off into a separate class that adds and removes seed discussions/posts outside of transactions during setUp/tearDown.

[ci skip] [skip ci]