Fixed compatibility with PHPUnit 6.x

tests/bootstrap.php

@@ -13,3 +13,5 @@ require_once(__DIR__ . '/../vendor/yiisoft/yii2/Yii.php');
 
 Yii::setAlias('@yiiunit/extensions/swiftmailer', __DIR__);
 Yii::setAlias('@yii/swiftmailer', dirname(__DIR__));
+
+require_once(__DIR__ . '/compatibility.php');

tests/compatibility.php

@@ -0,0 +1,12 @@
+<?php
+/*
+ * Ensures compatibility with PHPUnit 6.x
+ */
+
+if (!class_exists('PHPUnit_Framework_Constraint') && class_exists('PHPUnit\Framework\Constraint\Constraint')) {
+    abstract class PHPUnit_Framework_Constraint extends \PHPUnit\Framework\Constraint\Constraint {}
+}
+
+if (!class_exists('PHPUnit_Framework_TestCase') && class_exists('PHPUnit\Framework\TestCase')) {
+    abstract class PHPUnit_Framework_TestCase extends \PHPUnit\Framework\TestCase {}
+}