make compatible to phpunit 6

tests/bootstrap.php

@@ -12,3 +12,9 @@
 date_default_timezone_set('UTC');
 
 require __DIR__.'/../vendor/autoload.php';
+
+// B.C. for PSR Log's old inheritance
+// see https://github.com/php-fig/log/pull/52
+if (!class_exists('\\PHPUnit_Framework_TestCase', true)) {
+    class_alias('\\PHPUnit\\Framework\\TestCase', '\\PHPUnit_Framework_TestCase');
+}