MDL-48837 phpunit: Restore system default timezone

backup/util/helper/tests/cronhelper_test.php

@@ -33,6 +33,19 @@
  */
 class backup_cron_helper_testcase extends advanced_testcase {
 
+    /**
+     * @var String keep system default timezone.
+     */
+    protected $systemdefaulttimezone;
+
+    /**
+     * Setup.
+     */
+    protected function setUp() {
+        parent::setUp();
+        $this->systemdefaulttimezone = date_default_timezone_get();
+    }
+
     /**
      * Test {@link backup_cron_automated_helper::calculate_next_automated_backup}.
      */
@@ -457,4 +470,12 @@ public function test_next_automated_backup() {
         $this->assertEquals($expected, date('w-H:i', $next));
 
     }
+
+    /**
+     * Set timezone back to default.
+     */
+    protected function tearDown() {
+        date_default_timezone_set($this->systemdefaulttimezone);
+        parent::tearDown();
+    }
 }