updating tests by dropping PHPunit 4.6 support

tst/ModelTest.php

@@ -1,6 +1,7 @@
 <?php
 
 use Identicon\Identicon;
+use PHPUnit\Framework\TestCase;
 use PrivateBin\Configuration;
 use PrivateBin\Data\Database;
 use PrivateBin\Model;
@@ -10,7 +11,7 @@ use PrivateBin\Persistence\ServerSalt;
 use PrivateBin\Persistence\TrafficLimiter;
 use PrivateBin\Vizhash16x16;
 
-class ModelTest extends PHPUnit_Framework_TestCase
+class ModelTest extends TestCase
 {
     private $_conf;