Extract pure unit tests so that they can run fast

- Move to separate directory (base for a separate test suite)
- Inherit directly from PhpUnit
- Configure test suite with dedicated XML file

tests/unit/Settings/DatabaseSettingsRepositoryTest.php

@@ -0,0 +1,43 @@
+<?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\Settings;
+
+use Flarum\Settings\DatabaseSettingsRepository;
+use Illuminate\Database\ConnectionInterface;
+use Mockery as m;
+use PHPUnit\Framework\TestCase;
+
+class DatabaseSettingsRepositoryTest extends TestCase
+{
+    private $connection;
+    private $repository;
+
+    public function setUp()
+    {
+        $this->connection = m::mock(ConnectionInterface::class);
+        $this->repository = new DatabaseSettingsRepository($this->connection);
+    }
+
+    public function test_requesting_an_existing_setting_should_return_its_value()
+    {
+        $this->connection->shouldReceive('table->where->value')->andReturn('value');
+
+        $this->assertEquals('value', $this->repository->get('key'));
+    }
+
+    public function test_non_existent_setting_values_should_return_null()
+    {
+        $this->connection->shouldReceive('table->where->value')->andReturn(null);
+
+        $this->assertEquals('default', $this->repository->get('key', 'default'));
+    }
+}