Tests: remove prepDb workaround

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()`.

tests/integration/extenders/SettingsTest.php

@@ -17,8 +17,13 @@ class SettingsTest extends TestCase
 {
     use RetrievesAuthorizedUsers;
 
-    protected function prepDb()
+    /**
+     * @inheritDoc
+     */
+    protected function setUp(): void
     {
+        parent::setUp();
+
         $this->prepareDatabase([
             'users' => [
                 $this->normalUser()
@@ -35,8 +40,6 @@ class SettingsTest extends TestCase
      */
     public function custom_setting_isnt_serialized_by_default()
     {
-        $this->prepDb();
-
         $response = $this->send(
             $this->request('GET', '/api', [
                 'authenticatedAs' => 1,
@@ -58,8 +61,6 @@ class SettingsTest extends TestCase
                 ->serializeToForum('customPrefix.customSetting', 'custom-prefix.custom_setting')
         );
 
-        $this->prepDb();
-
         $response = $this->send(
             $this->request('GET', '/api', [
                 'authenticatedAs' => 1,
@@ -84,8 +85,6 @@ class SettingsTest extends TestCase
                 })
         );
 
-        $this->prepDb();
-
         $response = $this->send(
             $this->request('GET', '/api', [
                 'authenticatedAs' => 1,
@@ -108,8 +107,6 @@ class SettingsTest extends TestCase
                 ->serializeToForum('customPrefix.customSetting2', 'custom-prefix.custom_setting2', CustomInvokableClass::class)
         );
 
-        $this->prepDb();
-
         $response = $this->send(
             $this->request('GET', '/api', [
                 'authenticatedAs' => 1,
@@ -132,8 +129,6 @@ class SettingsTest extends TestCase
                 ->serializeToForum('customPrefix.noCustomSetting', 'custom-prefix.no_custom_setting', null, 'customDefault')
         );
 
-        $this->prepDb();
-
         $response = $this->send(
             $this->request('GET', '/api', [
                 'authenticatedAs' => 1,
@@ -158,8 +153,6 @@ class SettingsTest extends TestCase
                 }, 'customDefault')
         );
 
-        $this->prepDb();
-
         $response = $this->send(
             $this->request('GET', '/api', [
                 'authenticatedAs' => 1,