Shift 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/TestCase.php

@@ -104,7 +104,7 @@ class TestCase extends BaseTestCase
         }
     }
 
-    public function tearDown()
+    protected function tearDown(): void
     {
         Storage::disk('local')->deleteDirectory('testing-datastore');
         parent::tearDown();