Move test infrastructure to Testing namespace in src directory

php-packages/testing/src/unit/TestCase.php

@@ -0,0 +1,18 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Testing\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;
+}