Automatically set up Mockery for unit tests

- Use provided PhpUnit listener to enforce verification of expectations.
- Include Mockery's trait to auto-close Mockery after each test.

tests/unit/Settings/DatabaseSettingsRepositoryTest.php

@@ -12,9 +12,9 @@
 namespace Flarum\Tests\unit\Settings;
 
 use Flarum\Settings\DatabaseSettingsRepository;
+use Flarum\Tests\unit\TestCase;
 use Illuminate\Database\ConnectionInterface;
 use Mockery as m;
-use PHPUnit\Framework\TestCase;
 
 class DatabaseSettingsRepositoryTest extends TestCase
 {