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/AuthSSOTest.php

@@ -34,7 +34,7 @@ class AuthSSOTest extends DBTestCase
     private $original_auth_mech = null;
     private $server;
 
-    public function setUp(): void
+    protected function setUp(): void
     {
         parent::setUp();
 
@@ -468,7 +468,7 @@ class AuthSSOTest extends DBTestCase
         $this->assertTrue($a->authSSOParseGroups() === 10);
     }
 
-    public function tearDown(): void
+    protected function tearDown(): void
     {
         Config::set('auth_mechanism', $this->original_auth_mech);
         Config::forget('sso');