Update PHPUnit to 8.x (#11635)

* 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

* Update PHPUnit to 8.x

Part of Laravel 6 upgrade

* Bump php versions

Co-authored-by: Laravel Shift <shift@laravelshift.com>
Co-authored-by: Tony Murray <murraytony@gmail.com>

tests/ConfigTest.php

@@ -33,7 +33,7 @@ class ConfigTest extends TestCase
 {
     private $config;
 
-    public function setUp(): void
+    protected function setUp(): void
     {
         parent::setUp();
         $this->config = new \ReflectionProperty(Config::class, 'config');