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/TestCase.php

@@ -0,0 +1,20 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Tests\unit;
+
+use Mockery\Adapter\Phpunit\MockeryPHPUnitIntegration;
+
+abstract class TestCase extends \PHPUnit\Framework\TestCase
+{
+    // Ensure Mockery is always torn down automatically after each test.
+    use MockeryPHPUnitIntegration;
+}