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

Author: laravel-shift

tests/AutolinkTest.php

@@ -17,7 +17,7 @@ final class AutolinkTest extends TestCase
 {
     private $autolink;
 
-    public function setUp(): void
+    protected function setUp(): void
     {
         $parser = new Parser;
         $parser->addElementParser(new UrlParser);
@@ -29,7 +29,7 @@ public function setUp(): void
         $this->autolink = new Autolink($parser, $renderer);
     }
 
-    public function tearDown(): void
+    protected function tearDown(): void
     {
         $this->autolink = null;
     }