Set return type of base TestCase methods

From the [PHPUnit 8 release notes][1], the `TestCase` methods below now declare a `void` return type:

- `setUpBeforeClass()`
- `setUp()`
- `assertPreConditions()`
- `assertPostConditions()`
- `tearDown()`
- `tearDownAfterClass()`
- `onNotSuccessfulTest()`

[1]: https://phpunit.de/announcements/phpunit-8.html

tests/Feature/ArticlesApiTest.php

@@ -14,7 +14,7 @@ class ArticlesApiTest extends TestCase
     protected $faker;
 
 
-    public function setUp(): void
+    protected function setUp(): void
     {
         parent::setUp();
         $this->faker = FakerFactory::create();

tests/Feature/EventsApiTest.php

@@ -15,7 +15,7 @@ class EventsApiTest extends TestCase
     protected $faker;
 
 
-    public function setUp(): void
+    protected function setUp(): void
     {
         parent::setUp();
         $this->faker = FakerFactory::create();