updating tests by dropping PHPunit 4.6 support

tst/Persistence/PurgeLimiterTest.php

@@ -1,8 +1,9 @@
 <?php
 
+use PHPUnit\Framework\TestCase;
 use PrivateBin\Persistence\PurgeLimiter;
 
-class PurgeLimiterTest extends PHPUnit_Framework_TestCase
+class PurgeLimiterTest extends TestCase
 {
     private $_path;
 

tst/Persistence/ServerSaltTest.php

@@ -1,8 +1,9 @@
 <?php
 
+use PHPUnit\Framework\TestCase;
 use PrivateBin\Persistence\ServerSalt;
 
-class ServerSaltTest extends PHPUnit_Framework_TestCase
+class ServerSaltTest extends TestCase
 {
     private $_path;
 

tst/Persistence/TrafficLimiterTest.php

@@ -1,8 +1,9 @@
 <?php
 
+use PHPUnit\Framework\TestCase;
 use PrivateBin\Persistence\TrafficLimiter;
 
-class TrafficLimiterTest extends PHPUnit_Framework_TestCase
+class TrafficLimiterTest extends TestCase
 {
     private $_path;