Rework tests structure. Upgrade codeception to 2.5.3. Merge params configuration into app configuration.

common/tests/fixtures/MojangUsernameFixture.php

@@ -0,0 +1,13 @@
+<?php
+namespace common\tests\fixtures;
+
+use common\models\MojangUsername;
+use yii\test\ActiveFixture;
+
+class MojangUsernameFixture extends ActiveFixture {
+
+    public $modelClass = MojangUsername::class;
+
+    public $dataFile = '@root/common/tests/fixtures/data/mojang-usernames.php';
+
+}